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Blowback Head Space x25 Conversions

9K views 80 replies 14 participants last post by  1biggun 
#1 ·
While there's not much away from the muzzle end that can hurt in this hobby, out of battery or extreme head space can make some blood. I may put a trunnion in with toothpicks but the thought of an unsupported round going off in my face makes my skin crawl.

The open bolt sub guns fire before the round is fully chambered. A 7.62x25 sits 140 thousandths out of the chamber when it's all the way in. The gap between an AK74 bolt and the chamber face with a fully chambered round is 40 thousandths on a PPS43 barrel.

I've see no mention in other threads of machining the bolt face deeper although I may have missed it.

I think in terms of the bolt being held a few thousandths off the cartridge by the carrier on the trunnion but that leaves alot of cartridge unsupported.

Am I missing something here? :help:
 
#2 ·
You normally have the entire extractor groove exposed, plus enough of the case above that for the top of the extractor. Sometimes the barrel is brought back to the extractor groove and cut away for the extractor hook to rotate, like in a standard AK barrel.

Various designs also have reliefs cut into the bolt for the ejector. These usually don't compromise strength at the breach.

Some modern designs use a very narrow extractor hook, usually coupled with a spring-loaded in ejector built into the bolt face. The front of the bolt will socket into the back of the barrel. That's about as good as it gets.

Designs like the .30-06 Springfield and M1911 Colt didn't have particularly good support around the case head, so (since they were designing the ammo too) they made the case head thicker to handle the load.

My MAC-11 fires from an open bolt. The "firing pin" is a nub about .040 high. Almost by definition, it fires out of battery. It's not uncommon for it to blow the side of a case out ahead of the extraction groove, or eject pretty little fireformed cases that would fit nicely into a .380 Belted Magnum, if such a thing existed.
 
#3 ·
WInn,

ive built a lot of 7.62 subguns (just not in an AK) so im not sure if im understanding what your getting at...

do you mean using a 7.62 x25 in a chamber for 5.45 or something else?

reason im asking is because a x25 barrel is chambered as you mentioned to leave near 14oK "sticking out" this is what it was supposed to do. and there is no real issue there. thats why they are called "case poppers" they are well known for swelling and popping the case/brass as it was never intended to be a reloaded cartridge so it wasnt an issue... it was all about aid of extraction and speed at that time.

ive had case ruptures and blowouts in semi converted guns with x25 cartridge as your going from the bolt engaging and firing to a bolt slamming closed and sitting on it while a striker/hammer then ignites it... this in itself can lead to a higher probability of a blow out as its no longer a slight compression and near instant ignition and blow back all at the same time... its now a two stage operation.

the reason im sticking on this with you is that the 140k number is usually toward the feed side and not all the way around.

now if your just talking about an AK bolt head modification only... you already know the AZHONKEY is the best out there at setting these up.

im personally going to use a pps43 barrel and modified bolt on a blowback design for my AKx25 and then as long as the bolt face depth is the same as a pps43 bolt to match the chamber there shouldnt be any issue.

and if that doesnt make any sense at all you could always jsut delete this post. LMAO
 
#4 ·
On a PPSh 41 barrel I just copied the live round was .153 out BTW i checked it 6 zillion times LOL. I agree it sticks out a lot.

I Had planed on just duplicating the amount of protrusion of a 223 or 5.45 and set the bolt clearance at the minimum that will still give clearance. I measured the only 5.45 bolt I have and its .113 from the end to the face. a bulgy 223 is .121 and a rommy 7,62x39 is .110 taking the deepest bolt face of .121 and subtracting the amount of bullet protrusion got from a PPSH 41 barrel I come up with .032 bolt to breach clerance or .043 with the 5,45 bulgy bolt. IMOO thats to much and I agree with you.

I had not planed on using a traditional blow back barrel Im chambering my own. On the the one I started to do for a blow back I simply chamberd it the same depth as a 5.45 round protrudes problem solved. you are correct about being concerened with the amount of protusion using a PPSH 41 or 41 barrel and a unmodifed bolt.

I measured the bolt face depth on a PPS 41 bolt and it was .143 that would give .003 clearance on your barrel that had a .140 protrusion IMOO about right. I agree that the bolt depth needs to be addressed however I dont think the situation is that bad the bolt really dosent support the case only the chamber can do that. Id rather have the round in as far as possable. If it is safe to fire with .143 or in the case of the barrel I just measured .153 then even with a flat bolt face and no recess it should be safe. I also noticed a fair amount of slop compared to chambers I cut

personaly Id rather have the round seated deeper and have to use a extractor groove. and my builds will do so.
 
#5 ·
On a PPSh 41 barrel I just copied the live round was .153

I043 with the 5,45 bulgy bolt. IMOO thats to much and I agree with you.

you are correct about being concerened with the amount of protusion using a PPSH 41 or 41 barrel and a unmodifed bolt.

I agree that the bolt depth needs to be addressed
Yes -- that's exactly what I was wondering

WInn,

swelling and popping the case/brass as it was never intended to be a reloaded cartridge so it wasnt an issue...

ive had case ruptures and blowouts in x25 cartridge

you already know the AZHONKEY is the best out there at setting these up.
You guys are good but he's the MAN. The boy in El Paso is something too.

im personally going to use a pps43 barrel and modified bolt on a blowback design for my AKx25 and then as long as the bolt face depth is the same as a pps43 bolt to match the chamber there shouldnt be any issue.
OK -- now we're dancing together. The point is there's 40 thousands (1bg's #'s are the same) difference in the face depth.



That's about as good as it gets.

It's not uncommon for it to blow the side of a case out ahead of the extraction groove,
Double Damn!!!! Again
 
#6 ·
simply put there is a lot of case sticking out on many of the factory designs. none of the ppsh 41 or 43 use a extractor groove. Im not sure why, may be they eject/blow back better I dont know. but if Im chambering my own barrel Ill set the protrusion to the bolt clearance unless there is a reason not to. Im not a big fan of scrwed up brass even if you cant reload it. down the road surpluss may not be avaliable and reloading might be the only option so why design a gun to screw up brass un les there is a advantage?
 
#7 ·
The consensus is that this design splits cases.

I wonder if folks have been blowing the top covers off these conversions.




 
#8 ·
1biggun & Winn,

YES & YES...

the case hangs out by design. and yes the extractors are usually set up to work in that "space" of gap left in ..... instead of a channel cut in the trunnion.

no big deal for you to cut a small extractor channel in there as ive done this on a 7.62 sten for a guy...

same reason, the case stuck out too far and to get it into the chamber, he went a little deeper... which gave him a problem getting the extractor OVER the round being in too deep.

so i cut him a little channel in there and that now lets the extractor have enough room to be over the rim in his deeper chamber.

does it work in that sten? YES and it seems to be fine.

but it does "slow" the gun down in cycle speed even for a semi.

its just MO... but i think this is because the entire case is in the chamber and its not giving near as much help with the blowback ejection as it was with the tail end sticking out and swelling as it all forced backwards...

did not change the head depth on the semi sten bolt... left it as it was, he made it all up in the chamber.

to me changing the bolt depth would then have been putting "slop" in the extractor and gave you some movement there which would have left you a great chance of ripping off the edge of the cartridge if it actually stuck in the chamber... so it was better to have the solid end at the bolt and any play up front.

sdk
 
#9 ·
but it does "slow" the gun down in cycle speed even for a semi.

its just MO... but i think this is because the entire case is in the chamber and its not giving near as much help with the blowback ejection as it was with the tail end sticking out and swelling as it all forced backwards...
Interesting likely the case has to expand tight against the chamber then retract once pressure is reduced then there is less recoil to blow the action back. I know that one of the magnum pistols uses a grove in the chamber to keep the case in there untill the pressure has reduced before it blows back
 
#10 ·
I agree -- the best fix is to ream the chamber deeper. But damn it all, I'd hoped not to have to buy the friggin reamer. Clymer 30 Mauser? was that it?

I now understand why several conversions showed up with Krink hinged covers -- they don't get blown off!!

I also now understand why the sub gun components surrounding the chamber were so heavy.
 
#11 ·
ha ha......... nothing slides by you two does it?

its one of the reasons i like this forum.... good natured people who like to actually BUILD and talk about building guns.

the reamer for 7.62x25 TOK is not cheap. ive "borrowed" one before.... and ive heard talk that some of the "real" fancy machinists use that another reamer but it is a shade different and then its a "feel" thing.... which would kinda defeat the point of making the change if you were still guessing at it.

and yep thats why the subby's are so heavy. takes a heavy bolt for that hot little round, which is why im wanting to use a PPS43 or 41 bolt on the modified carrier... to gain that weight.

gonna be a winter time project when things are slower...

sdk
 
#14 ·
ha ha......... nothing slides by you two does it?

its one of the reasons i like this forum.... good natured people who like to actually BUILD and talk about building guns.

the reamer for 7.62x25 TOK is not cheap. ive "borrowed" one before.... and ive heard talk that some of the "real" fancy machinists use that another reamer

sdk
"real" fancy machinists -- Well Sean, I guess that leaves me out!


Two comments, don't know if they will help though.

1-you have to remember the PPSH41 barrel is chrome lined, that might cause problems using a reamer?
No -- I didn't know that. Like an idiot, I looked at the ones I've got and thought how lucky I was they were like new. That does however save me the reamer cost!

Now I'm guessing but this gap might close a little more when the heavy bolt slams against the round pushing it further into the chamber some.
I think it does close a little more but from the comments above still has a tendency to blow cases. I want a gun my middle school children can shoot.



Winn, that is the reason that I chose not to use a subgun barrel that I had. I can say for a fact that a .308 blank that is chambered with a 30 Mauser reamer will shoot 7.62x25. I just cut the chamber until there was .005-.006 clearence between the AK bolt face (no extractor) and the barrel face using a dummy round. Then set everything so that there was about .002 headspace.
Thanks Gunter, that is the solution I had in mind. A casing supported is a casing that won't split.

I wonder now, given the chromed chamber, if I should look again at cutting into the '74 bolt face. Dutigaf's comment above is well taken, the slop around the extractor could cause problems. Maybe.. I'll assume.. (boy this is getting me in deep) that the blowback operation would minimize extractor related problems.

Or, and I really don't want to go here, I could tig material to the breech face, or weld on a washer, then turn it down to get Gunter's numbers.

What do you guys think?
 
#12 ·
Two comments, don't know if they will help though.

1-you have to remember the PPSH41 barrel is chrome lined, that might cause problems using a reamer?

2- I dug out my unmodded PPSH41 bolt and inserted a spent case, the bolt surrounds the base of the case up to the front of the extraction groove, in the pic this is about where the red line is. Click on the pic for a larger image
I can't compair it to the barrel because it's not here right now, but I'll do what I can when it is. ;)
Now I'm guessing but this gap might close a little more when the heavy bolt slams against the round pushing it further into the chamber some.
 

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#13 ·
Winn, that is the reason that I chose not to use a subgun barrel that I had. I can say for a fact that a .308 blank that is chambered with a 30 Mauser reamer will shoot 7.62x25. I just cut the chamber until there was .005-.006 clearence between the AK bolt face (no extractor) and the barrel face using a dummy round. Then set everything so that there was about .002 headspace.
 
#15 ·
I have about $150 into my reamer with the extra pilots that I got to do from .307-.310 bores. I cant cut a chrome chamber .

the 30 mauser is slightly differant but it will work it may not however be as accurate or treat the brass as good if reloading comercal is a concern.

from what Ive read and already seen most of the surplus splits necks I have several of the rommy split from were its crimped in my 45-70 addapter. I have no neck bulging but in the addpter its flush with not protrusion.

I dont think cutting the bolt face will offfer any real advantage as the case is still unsuported by the chamber. the bolt face is larger than the round and the round tapers any way. it may help redirect a failure but it is not going to reduce or prevent one. alsoif the bolt face is cut it will place the ejector to far forward un less its modifed.

I can cut or lengthen a chamber on a Non chromed lined barrel and Im looking for work. I have no idea how many chambers a reamer will do before it needs to be sharpend or replaced. If some one wants some thing done PM me. I was going to sell some chamberd blanks but Im looking for a cheap source of blanks or take off barrels. I have a few remington n winchester take offf barrels but they were not super cheap so it is hard to offer a cheap chamberd blank.

My build will have a deeply seated chamber and a extractor cut. I want to be able to relaod brass in the future if the surpluss dissapears or gets out lawed. also accuracy is going to be better with a fully seated round that isint bulging or bursting. as long as it ejects Im ok.
 
#16 ·
I just cut the chamber until there was .005-.006 clearence between the AK bolt face (no extractor) and the barrel face using a dummy round. Then set everything so that there was about .002 headspace.
sounds right to me. :) I am doing a gas operated AMD 65 like that now. well at least the barrel I have to many projects and no cash.
 
#17 ·
Here's a 9mm winchester ranger (black talon) that ballooned at the feed ramp bevel. The back side where there is no bevel is fine. It happened when a temporary recoil spring took a set and did not close the bolt completely. I was suprised that the case balloned out that much but did not rupture. I wonder if a steel case would do the same or if thats why people are reporting so many split cases with the 762x25? I would try to get as little of the case exposed as possible.
 

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#18 ·
Im wondering if to light of a bolt is allowing some of the builds to have issues as well? possably being blown part way out of the chamber before the pressure drops off??

Id like to come up with a way to prevent a out of battery fire. some sort of a acuater that only allows the the pin to be exposed whn the bolt is fully forward or or a trigger block that does the same. It cant be that hard. to be honest a the thought of a slam fire or a accidental douple or bump fire that lets the hammer ride the bolt home or relase on it way home or to fire on a partualy chamberd round scares the hell out of me. I wonder if there is a way to profile the bolt or hammer so it can only hit the pin at the vary last part of the bolt travel?? sort of l;ike the tail on a ak carrier but one that the hammer would hit untill the carrer is at least 90% forward. there has to be a easy way.
 
#20 ·
Im wondering if to light of a bolt is allowing some of the builds to have issues as well? possably being blown part way out of the chamber before the pressure drops off??

Id like to come up with a way to prevent a out of battery fire. some sort of a acuater that only allows the the pin to be exposed whn the bolt is fully forward or or a trigger block that does the same. It cant be that hard. to be honest a the thought of a slam fire or a accidental douple or bump fire that lets the hammer ride the bolt home or relase on it way home or to fire on a partualy chamberd round scares the hell out of me. I wonder if there is a way to profile the bolt or hammer so it can only hit the pin at the vary last part of the bolt travel?? sort of l;ike the tail on a ak carrier but one that the hammer would hit untill the carrer is at least 90% forward. there has to be a easy way.
most of the subgun bolts that get converted to semi end up lighter than what they started out at. They also don't have the extra kenetic energy of the bolt moving forward and the fixed firing pin setting off the round before the bolt closes.
As far as profiling the bolt so it can't set off a round, I'd almost think it should be as round as possible to ease the bolt down. Otherwise it would close faster on a 90 degree edge I'd think. The ar15 firing pin I use in my suomi conversions is set up as an inertia firing pin. With the hammer all the way down there is still at least a 1/16" gap before the firing pin starts to touch the primer plus the heavy firing pin return spring. When I was testing the design I took the disconnector out of the trigger group and tried to get a test round (no powder hole in bullet nose) to set off a primer or leave a dent in the test round primer. I settled on a combination that gave reliabale ignition, but would only slightly mark a soft remington primer (about the same amount as an ar15 does chambering a round) in case of hammer follow.
Using a different design or modifying the disconector on an ar15 or ak47 so that it won't release the hammer from the disconnector to the trigger unless the bolt is fully closed would get you this safety. Figuring out the linkage would be the pisser there, as well as making it legal like 1Biggun said. The remington 870 style hammer and sear I'm using could very easily be adapted to for this and actually has theat feature in the comercial remington guns.
 
#19 ·
I see a way it cold be done with a rod piviots that runs under the rail that as the bolt moves back would make it impossable to pull the trigger that that once the bolt returns the rod wold pivot back with spring pressure to allow it to release again .. I have no real idea hoow the full auto parts work or if this might some how duplicate them in some way and be considerd illeagle but I can see it being made to work with a AK trigger system. it wouuld not affect the disconector it would simply act like a saftey unless the bolt was closed. it would have a zone were the bolt is setting the trigger were the trigger could be pulled but as the bolt returns forward and the hammer is not being hled down by the bolt it would again bar the trigger. I think it would workit would require three pieces a bar a pivot and a spring.
 
#21 ·
Ive got the design in my head it would have to be with a custom bolt but I plan on doing that any way. The thought of one over size case or jam due to dirt ect really bothers me. I have this happen in a few SA 22lr,s the round isint all the way in the barrel. no big deal on 22lr but it could be bad with a the much hotter x25. I suspose a looser chamber would be safer as far as bad ammo or dirt.
 
#22 ·
lots of great input in this thread!

a very good adult discussion. congrats to all for keeping it civil!

back on the topic...

first thing:

WinnR:

i just dont consider the x25 round a good "kids round" and dont let my kids shoot any of my x25 guns. its just WAY too hot and the loads are just crazy in their differences of power/speed. and ive built way more x25 guns than im ever gonna admit or post.

1biggun:

someone mentioned the chrome barrel thing, the barrel i took deeper was NOT a chrome barrel it was just a 16" regular x25 barrel from IO that we stuck in a sten build.. so taking it a hair deeper was as easy as you mentioned and then adding that extractor "notch".

Moleman:

im gonna use the ppsh or 43 barrel and bolts combo for exactly the reason you mentioned.... its DESIGNED to have that space and functions perfectly that way.

the bolt weights are very important, we found out early in the ppsh41 conversions and then the 43 that you had to have all that weight for a short barrel and that a little more doesnt hurt if your using the 16" barrels.


the x25 is a great caliber to play with. its dirty, loud, smoky, fiery and cheap to shoot.


just look at my P7 video and see what i mean, that is in x25 with a 16" barrel and the factory epth of chamber and bolt head depth is set by the 9mm AR bolt head. it works perfectly and runs like a raped ape....

but it is just like all the FA x25's and just kills the cases.

the cartridge was just never meant to be reloaded.

sdk
 
#23 ·
I dont let my kid shoot any of my semi autos yet just to many things to go wrong. he will learn to be a better shot becuse of it. I do let him shoot the rugger 10-22 on ocasion but its very supervised. I am going to build some sort of bolt action or break action in x25 that my kid can shoot cheap but it will be completly safe and over built.

the bolt weights are very important, we found out early in the ppsh41 conversions and then the 43 that you had to have all that weight for a short barrel and that a little more doesnt hurt if your using the 16" barrels.
good to know do you have a good average weight for a blow back bolt or does the spring tension really make it impossable to nail down??
 
#24 ·
or if thats why people are reporting so many split cases with the 762x25?
How are they splitting, length wise or around it?
I ask because I got Bulgy and Romy stuff, the Bulgy sometimes gets splits/cracks from the front to the taper, I think it's just the brass is brittle in this area. I never had any issues with the Romy stuff, but then that is steel cased.

the bolt weights are very important, we found out early in the ppsh41 conversions and then the 43 that you had to have all that weight for a short barrel and that a little more doesnt hurt if your using the 16" barrels.
Thanks, that info might be useful to me soon.
 
#25 ·
good to know do you have a good average weight for a blow back bolt or does the spring tension really make it impossable to nail down??
If he can't answer, I'll weigh some of the bolts I have to see if there is an average weight between then. The spring does matter too, but usually the stock spring will work on a modded bolt too.
 
#26 ·
yeah i can go get some of the bolt weights tomorrow..

but i'll give you the ppsh 41 bolts as modded by SHerman for his semi conversion...

the first one we did was 564 grams after the mods and rod re insert...

the next one was done the same way and came in at 595grams

and we had one that was near 620 and it was a tad too heavy.

i'll get you the original weights tomorrow if someone doesnt beat me to it.

oh and yes the return spring does figure into the bolt weight but ive always just liked the heavier bolt and counted on just enough spring to chamber the round ... not really using it as a holding force....


my cases have always split from neck to nose and they all swell around the base.
 
#27 ·
I wonder if the x25 chamber reamers are all on the large size? Kinda like the chamber is oversized like on the M1 carbine so it will work well when its very dirty. Every carbine I've had swells the case slightly at the web.
 
#28 ·
OK -- I'm a happy camper!!!! Cut the chromed chamber deeper.

With God's own collection of reamers I gleefully assumed the taper would be cut big and the only thing would be to lower the shoulder and casing ledge .030".

It worked!! Hand reamed, lots of cutting oil. Dycemed up a yugo cartridge to see where it hit, found the two reamers I needed (if someone wants I can go out and measure them) by dropping them in the chamber and did it by feel. Cleaned up the mess and fit alot.

The extractor hits the breech face and I'll have to mill that out but I've got a snugged up bolt and cartridge now.
 
#29 ·
I wonder if the x25 chamber reamers are all on the large size? Kinda like the chamber is oversized like on the M1 carbine so it will work well when its very dirty. Every carbine I've had swells the case slightly at the web.
i do know that the 41 barrel I just copyed the round was fairly loose in the chamber compared to the chamber I cut that had no side to side play. I shot about 50 rounds through may addapter and had about 4 cases that split from the pin like crimp down I shooting rommy 80,s stuff that I have read is susposed to be the least likely to split. i noticed differances in recoil as well this stuff is not very consistant. the addapter has no protrustion and about .0025 head space on the average I hade no bulging as the case is 100% suported by the chamber but as mentioned some splitting . nothing that wold cause a issue but I havent shot 500 rounds either. brittle brass IMOO
 
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