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8mm AK... how hard could it be??

8K views 55 replies 11 participants last post by  1biggun 
#1 ·
I've converted an 8mm bolt action to a .308 simply by changing out the barrel. The rounds fed fine from the magazine, bolt face matched... no changes anywhere else.

Maybe it's apples and oranges. But it makes sense to me that I could concievable buy me a .308 Saiga, turn a 8mm barrel for it from a blank (wich are fairly common and cheap, I might add)... and that's that. Bolt and everything should work fine, no? Probably need a bigger mag well... mag issues?? An 8mm round is too long to stuff into any .308 mag I suppose?? Is the Yugo 8mm AK the only weapon with a mag that would work for this?? Just curious as to other ppl's thoughts about this as a possible conversion. There's gotta be an alternative to dishing out 1K for a bag'o parts to build into an 8mm AK. Anyone? Thoughts? Opinions?
 
#27 ·
It would be nice to determine CUP pressure, and if it is close to a .308 or a 7.62x39, then I would say the bolt has enough meat out of the box. Would definitely be something I would consider building from a heavy RPK-style receiver just for that extra bit of security. Of course, the "hang time" on a semi is pretty low, so I think the pressure would bleed off fast enough that you would be OK.

Judging from the M76 pics, I am confident that it would work, would just need to get that CUP pressure info before you begin.
 
#28 ·
Im using a yugo m70 bolt on my 308 conversion. if its heavy enough for that it should handel a a 8mm with a lower cup pressure. some were on this site I read that the M76 ( 8mm mauser) was a 2 lug bolt and trunion set up. bolt length is a issue with the 8mm round I was thinking of extending the bolt and pin out some how. as far as grinding the trunion it would only need to be widend slightly to get a mag to fit it if at all with a m14 mag see sangs posts on the 308 conversion forum. moving the trigger guard and latch is almost a given. I am planing to move the latch piviot back aprox a 1/4" and rework the spring (well the whole guard) and take another 1/4" out of the guard its self this will leave pleanty of finger clearance. ( this on the 308 conv) Bolt face and extracter will need to be opened up as well. the 8mm round is only 6mm longer than the 308 if one were to take say 2mm out of the trunion and 4 more out of the trigger guard mag latch area it the rest would be the same as a 308 conversion. I make my own recivers so moving the trigger group back or the trunion forward is a option as well. If a longer bolt was avaliable I would just make a 3/4" longer reciver moving the front trunion forward. (I made my jig extra long already HEE HEE) as far as gas block and gas port hole size I would go with a L1A1 adjustable gas block and tune it to work with what ever ammo you find cheap. I would not use a Sagia 308 as a starting point when a Yugo kit is only $100 dollars and you are going to not use the barrel. why wreck a 308 Saiga when the whole gun can be built for less than what the Saiga cost to start with? Yugo kit aprox $100 8mm mauser barrel on ebay $15 or less US parts$60 Mags $2.00-$25 reciever blank $15depending what you go with say aprox $250 for all. having a 8mm or other custom calibur you built yourself priceless
 
#29 ·
FYI:

http://www.thehighroad.org/showthread.php?t=34068

7x57 Mauser: 46,000 CUP
.303 British: 45,000 CUP
8x57 Mauser: 37,000 CUP (SAAMI, artificially limited for commercial liability)
8x57 Mauser: 50,000 CUP (As loaded in Europe, commercial and military)


http://www.websitetoolbox.com/tool/post/surplusrifle/vpost?id=6429&trail=15

150 Grain Speer Soft Point:
CASE Winchester Primers: Winchester LR
COL 2.945"

IMR Powders:

4831 47.0 2325 fps 36,800 CUP
4350 43.0 2315 fps 36,800 CUP
4320 36.0 2270 fps 36,900 CUP
4064 36.0 2305 fps 37,000 CUP
4895 36.0 2310 fps 37,000 CUP
3031 34.5 2335 fps 37,000 CUP
4198 28.5 2225 fps 37,000 CUP



http://www.thegunzone.com/30cal.html

7.62x51 = +/- 52,000 CUP.

Moral of the story - "Military" loads are around 50,000 cup. Both should be doable in a .308 Saiga style build (thicker receiver). The bolt head concerns me too. I think you would be OK, but that extra metal on a Saiga .308 conversion would make me feel much better when I put that up to my face when shooting...
 
#30 ·
hcpookie said:
Moral of the story - "Military" loads are around 50,000 cup. Both should be doable in a .308 Saiga style build (thicker receiver). The bolt head concerns me too. I think you would be OK, but that extra metal on a Saiga .308 conversion would make me feel much better when I put that up to my face when shooting...

...see, that's what I was thinking.. peace of mind. The bolt on a standard M70 might not fly apart today.. .might not fly apart tomorrow... but it seems to me they designed it beefier on the larger caliber versions for a reason. M70 trunnion.. no problem with. It's all about the bolt.

As far as a lot of money put into a conversion vs scratch build.. I don't think I'd be saving money starting out from scratch with a dark-bore M70 kit. I'm thinking by the time I get done screwing around trying to lengthen a bolt.. make this work, make that work.. it will have just been easier to rebarrel a .30-06 or .308 Saiga in 8mm and work out a mag that will feed... half the engineering is already done for me. Now... if I could lay my hands on a bolt, carrier and trunnion for an 8mm, .30-06 or etc... then I'd say sure, sign me up for a dark bore kit.

Anyway... still just working it out on paper before I commit to actually working on this... all this information is really helpfull btw. I've went ahead and ordered some mags from IMA to see how they look as a sign of my being comitted to finding a solution.
 
#32 ·
Thanks! I've noticed that it's not so much that the lugs are that much bigger.. just the bolt head is longer so that the hole for the extractor take-down (or whatever you call it) is in front of the lug instead of through it/under it. I'm guessing that makes it stronger? Perhaps testing at the factory deemed it necessary??
 
#33 ·
I was only comparing prices, not time involved, and I know time is money. easy'r to do sure. persionaly If I had a 30-06 saiga I would not touch it. what would you gain? even max 8mm loads will not touch a 30-06 in the (150 -180 grain range) or a 308 for that matter my feeling is that you would be making a good gun worse by converting a saiga 30-06 to a 8MM. (just my opinion not a slam or a argument) to me the most compeling reason to do a conversion is the cheap Yugo kits with bad barrels (what else are you going to do with them) and the unqueness of a one off firearm. I am not saying a 8mm would not be cool as hell ,but I would still be asking why and what did you do with the barrel. If you are going to convert a Saiga to an assult style weapon there are those costs as well. Most of my builds start life as a pistol for hunting thats what got me into the AK, this also rules out starting with a saiga unless I want to replace reciver as well. a 8MM pistol using a 220 grain slug would sure knock the hell out of a white tail at 150 yards. another good site for load info is http://www.hodgdon.com/data/rifle/857mmmau.php
 
#35 ·
1biggun said:
I was only comparing prices, not time involved, and I know time is money. easy'r to do sure. persionaly If I had a 30-06 saiga I would not touch it. what would you gain? even max 8mm loads will not touch a 30-06 in the (150 -180 grain range) or a 308 for that matter my feeling is that you would be making a good gun worse by converting a saiga 30-06 to a 8MM. (just my opinion not a slam or a argument) to me the most compeling reason to do a conversion is the cheap Yugo kits with bad barrels (what else are you going to do with them) and the unqueness of a one off firearm. I am not saying a 8mm would not be cool as hell ,but I would still be asking why and what did you do with the barrel. If you are going to convert a Saiga to an assult style weapon there are those costs as well. Most of my builds start life as a pistol for hunting thats what got me into the AK, this also rules out starting with a saiga unless I want to replace reciver as well. a 8MM pistol using a 220 grain slug would sure knock the hell out of a white tail at 150 yards. another good site for load info is http://www.hodgdon.com/data/rifle/857mmmau.php
I hear ya.. doesn't make any sense to mess with a perfectly good rifle, does it. What got me thinking along those lines was that a Saiga is FAR cheaper than what they're asking for an M76 de-milled rifle these days... so I started to think about how I could do it. Re-barrel/fit mags. Cheapest while still being the easiest. If the picture of that 30-06 Saiga is accurate it looks like it may be on a milled reciever anyway.. so there's a monkey wrench in the works. Tell you what... if I knew for sure a standard M70 bolt would cut the mustard and not eventually fly apart on me.. I'd work with that for sure. What were the pressures on a 7.62x39 vs 8mm again??
 
#37 ·
I'm really, honestly, truely not trying to be a wet blanket but I honestly think you guys are asking for trouble trying to go from a 7.62x39mm to a .308 or 8mm. Much higher pressures and more powerful rounds and the Yugoslavians (and Russians and Israelies and Finns) came out with rifles specifically adapted to them.
 
#38 ·
Templar said:
I'm really, honestly, truely not trying to be a wet blanket but I honestly think you guys are asking for trouble trying to go from a 7.62x39mm to a .308 or 8mm. Much higher pressures and more powerful rounds and the Yugoslavians (and Russians and Israelies and Finns) came out with rifles specifically adapted to them.
I have to agree.

Unless you are reloading your ammo with those lighter bullets, the CUP pressure is going to be pretty high (see my previous post).

Most all of that cheap ammo is built for Mausers and/or some type of MG - both of which are heavier-duty platforms than an AK action.
After looking at those CUP numbers, I don't like the idea of using cheap ammo in a 7.62x39 kit. I would only use hand loads on the light side. Most sites I visited imply the 8mm is regarded as 'close' to 30.06 on the hot side. Maybe converting a 30.06 Saiga would be the best?

Reloading ammo would negate the main reason I would do t is - a *cheaper* alternative to the M76.
 
#39 ·
hcpookie/templar...

No worries about wet blankets.. it's just a conversation to satisfy some curiosity. Where do you think the failure is most likely to come from off an M70.. the bolt, trunnion.. all of the above? I guess what I want to know is.. .what part of the original 8mm rifles was specifically designed for it? My .308 Saiga front trunnion with only one bulge in it doesn't look nearly as beefed up as a standard M70 trunnion with two.. seems like the M70 trunnion could handle it. It looks as though the 8mm bolt and the M70 bolt are pretty close despite the longer bolt stem.
 
#40 ·
I belive the veper uses a rpk trunion in it's 308 version and as far as I can tell the yugo m70 trunions and the rpk are the same. If some one know's of a differance I would like to know. I have seen some reloading sites that show the 7.62x39 loaded up to 50,000 CUP pressures with no prolems http://www.ak-47.net/ammo/reload2.html & http://www.chuckhawks.com/7-62soviet.htm.
http://www.hodgdon.com/data/rifle/223rem.php In fact at over 51,000 cup the 223 is as high as a 308 or a 30-06. the m77 (308) and m76 (8mm) appear to use a m70 style trunion. a lot of this has been coverd in the 308 conversion forum that I started several monthes ago. Now I relize that chamber pressure and recoil are two different things and that a 8mm with a 200 grain bullet with only 39,000 cup is going to kick a hell of a lot harder than a 223. this is the force that works the reciver ,stock, rivits screws your shoulder ect. the chamber pressure is what works the bolt and trunion lugs. If I have this wrong let me know.. this is what let's me belive that a 308 or 8mm on a m70 stamped bulged trunion is safe. The milled version it thinner than the stamped m70 yet thicker than Yugo stamped.
 
#41 ·
1biggun said:
I belive the veper uses a rpk trunion in it's 308 version and as far as I can tell the yugo m70 trunions and the rpk are the same. If some one know's of a differance I would like to know. I have seen some reloading sites that show the 7.62x39 loaded up to 50,000 CUP pressures with no prolems http://www.ak-47.net/ammo/reload2.html & http://www.chuckhawks.com/7-62soviet.htm.
http://www.hodgdon.com/data/rifle/223rem.php In fact at over 51,000 cup the 223 is as high as a 308 or a 30-06. the m77 (308) and m76 (8mm) appear to use a m70 style trunion. a lot of this has been coverd in the 308 conversion forum that I started several monthes ago. Now I relize that chamber pressure and recoil are two different things and that a 8mm with a 200 grain bullet with only 39,000 cup is going to kick a hell of a lot harder than a 223. this is the force that works the reciver ,stock, rivits screws your shoulder ect. the chamber pressure is what works the bolt and trunion lugs. If I have this wrong let me know.. this is what let's me belive that a 308 or 8mm on a m70 stamped bulged trunion is safe. The milled version it thinner than the stamped m70 yet thicker than Yugo stamped.
You're on the right track, and please don't get me wrong - I *DO* believe it can be done, given certain considerations...

This is NOT something I'd buy a box of cheap '60s surplus Turkish tin-canned ammo and start blasting away. If it were MY face that was making a cheek weld onto that converted build, I would make darn sure that the ammo was a known factor - hand-loaded on the light side of the chart with lighter bullets.

Personally I think .308 is the highest CUP of any of the calibers I've looked into. However we have a .308 conversion (Saiga) to use as a model. Honestly I can't think of any 8mm model other than the M76.

To restate my conclusion, handloading "good" ammo makes this more expensive and therefore counter-productive IMO. I've got the bug for an 8mm MG42 only because that MG-grade "blaster" ammo is so darn cheap! :thumbup1:
 
#42 ·
Hcpookie we are on the same page. I would not trust a lot of that old 8mm ammo out there plus I hear that it is really dirty and corrosive. I am going to do the 308 first plan on shooting it with NATO loads for fun and hand loads for hunting. If I get the mag and feeding working I would probably build a 358-08 for a heavy pistol round http://www.hodgdon.com/data/rifle/358win.php. and a 243 for varmits. the only differance would be the barrel as they all use the 308 parent casing. 7mm-08 is another good choice as well. just bought 3 more yugo gits it beter work.
 
#43 ·
I think you need to excersise some caution running ANY surplus 8mm through ANY semi-auto (sans MG's)... but that's just common sense. I would think you'd be fine however, with S&B or some other commercial load. Especially that stuff I think Winchester puts out?? Heard it's loaded on the lower end of the spectrum.

All that aside... sounds like the most critical area would be the bolt and probably the carrier, I believe the trunnions are the same.
 
#44 ·
See I haven't even looked at what's out there other than surplus.

I think the bolt lugs are the most critical area, as well as the chamber area. Winn's toothpick test has proven that the trunion experiences less force than for example the rear stock tang/rear trunion area due to recoil forces. The bolt lug lockup is the area that I would be most concerned about. A properly headspaced gun shouldn't expose any area, but if you have a weak cartridge it is gonna blow out at the back. A ruptured cartridge in the chamber is no sweat - you get a case extractor and you're back in business. That "open" area is IMO the danger zone. Unless you have a low-quality barrel. :dunno:
 
#45 ·
Well.. according to grasshopper, the 8mm bolt locks up in an M70 trunnion. Visually the lugs look the same (only seen the pictures, haven't truly mic'd it)... it just appears that the bolt face/head is longer than an M70's.. putting that hole for the extractor farther away from that lug. Knowing that 8mm's have been built (as well as .308's, and 54r's) I would think that if you had an 8mm bolt and carrier (for good measure) with an M70 trunnion.. you'd be good to go, safety wise. At least as safe as any other 8mm.

Ya... the winchester 8mm hunting ammo is loaded very mild from what I understand. The S&B is loaded more to European specs with a 196gn bullet. I guess it would help to know what the Yugo's shot through their M76's... but I suspect it was the same stuff they ran through their M48A bolt guns.
 
#46 ·
hcpookie said:
See I haven't even looked at what's out there other than surplus.

I think the bolt lugs are the most critical area, as well as the chamber area. Winn's toothpick test has proven that the trunion experiences less force than for example the rear stock tang/rear trunion area due to recoil forces. The bolt lug lockup is the area that I would be most concerned about. A properly headspaced gun shouldn't expose any area, but if you have a weak cartridge it is gonna blow out at the back. A ruptured cartridge in the chamber is no sweat - you get a case extractor and you're back in business. That "open" area is IMO the danger zone. Unless you have a low-quality barrel. :dunno:
I'll have to take a look at my Hakim and see how much cartridge hangs out of the chamber... seems to me I remember thinking it seemed like a bit much, but there you have a professionally engineered 8mm semi. Apples and oranges when compared to an AK, I know.

I picked up some MG-13 mags and it looks as though they could work after you mod a front and back lug. The mag itself is thinner than a standard AK mag, wich would give you some room to work with said mods. (anyone know where I can get junk AK mag bodies to cannabalize for dirt cheap??) I have a fixed stock Yugo kit to work with on it's way, and I have a possible line on an 8mm bolt and carrier... that part is key. I'm sure a Saiga .308 could be made to work... but do to the fact you'd have to chop a rifle up, the trunnion is sorta Saiga specific in that you'd have to use that Saiga reciever.. wich would mean a whole lot more work as far as moving everything back to accomodate that mag. Ah well... we'll see. If nothing else, I'll have some extra parts to peddle if it doesn't look like it'll fly.
 
#47 ·
My plans are: A M70 Yugo parts kit, a Coldsteel Solutions blank M70 receiver, and modified MG-13 magazines.

Push the FCG back about 1/4 inch, and the crosspin moved back as well to clear the MG-13 magazine. The hammer may have to be grounded down to clear the crosspin.

I already have everything, I just need a new shop so I can start building.

I gave away the three cases of hot 8mm ammo I had, so I should be O.K. ;)
 
#48 ·
I was also looking at those Romak III PSL blanks by Coldsteel... I was thinking that it should have the correct length with nothing getting in the way, and they've got a nice long mag dimple. I have no idea how different the bulges are from M70 to Romak though.

Just an FYI...
I took the micrometer to the barrel of a 8mm Yugo M48A and one of my other Yugo M70 kits.... the barrel diameter, at least towards the muzzle.. is identical. Very cool... means I should be able to utilize the gas block and front site from an M70 kit as well as the RSB.

Anybody get a good look at that grenade cut-off switch?? Anyway that can be cleanly modified into an adjustable gas block??
 
#49 ·
SA58 said:
Anybody get a good look at that grenade cut-off switch?? Anyway that can be cleanly modified into an adjustable gas block??
I think Cephus or Winn did that... definitely looks doable.
 
#50 ·
CUP and PSI

There seems to be a bit of confusion on CUP and psi around here. Psi is a more modern measure of pressure. CUP and LUP were the standard for many years, and you will find references to it in handloading books for some time to come. CUP and LUP are copper and lead crusher pressures respectively and are actually the same measure of pressure, they are interchangeable. Psi and CUP/LUP are not interchangeable, nor is there a conversion factor between the two. CUP is a caliber specific measurement. The same CUP in one caliber is not the same as the same CUP in another caliber. 52,000 CUP in a 6mm Rem is not the same as 52,000 CUP in a 223. The psi difference between the two is 10,000psi. I know this makes no sense so just check out the attached table. It came from a Hercules handloading pamphlet, it?s not BS.
You can?t really say how much pressure an action can take, you need to calculate bolt thrust. The amount of reward thrust placed on the locking system is what determines what an action can take. When fired, a case pushes backward in response to pressure like a hydraulic piston. The total amount of reward thrust is determined by pressure and area for the cartridge base. Would you rechamber a crappy old 1 MK3 Enfield and rechamber it to 45-70, 45 ACP, 7.63x39 or even .223 Rem? Probably not, but lets do some quick math. To calculate bolt thrust you need to know the max cartridge pressure in PSI not CUP/LUP, and the cartridge case base diameter, which is the diameter of the case above the rim or extractor groove, not the head or rim diameter. You can find both in a good loading manual such as Speer or on line at Ammo Guide. Using the case base diameter, find the area of the case in cross section, which is what you see if you stare down the neck of a fired case. The formula is (radius) squared * 3.14 or you can use (diameter) squared *.785, either will get you the roughly the same answer. Once you get the area multiply it by the max pressure in psi not CUP. This is the max amount of rearward thrust placed on the locking system in pounds. Here are the case dia and max pressures of the cartridges I mentioned above.
Cartridge Case base dia Max pressure Rearward thrust lbs.
303 British .455 49,000 7963
45-70 .470 28,000 4855
9mm para .391 35,000* 4200
45 ACP .476 22,000* 3913
223 Rem .375 55,000 6071
7.62x39 .438 45,000* 6777
* pulled these off the web, couldn?t find max in psi in my books, only CUP.

Take a good look at the cross section area and max pressure columns, there is a relationship between them and bolt thrust. If the pressure and area are higher then the total thrust is higher, reduce either and the thrust is lower. The 223 has the highest pressure but not the highest thrust. The 45-70 has the highest area but not the highest thrust. Note that the 45ACP and 45-70, have similar head diameters and pressures, thus they produce similar (sort of) rearward thrust. Also note that the 7.62x39 actually produces a higher thrust than the 223. Of interest to those doing pistol caliber conversions is that the 9mm actually puts more stress on the locking system than the 45ACP, but in a blow back system that?s a whole nother can of worms. By the way, As far as I know the 1 MK3 Enfield has been reliably chambered in all of the above cartridges except 223(case head is too small for the bolt face).

I also worked out the bolt thrust on the following cartriges for reference.
Cartrige Case base dia Max press psi Bolt thrust in lbs
7.62x39 .438 45,000* 6777
8mm Mauser .470 35,000*Saami 6069
30-06 .470 60,000 10404
308 .470 60,000 10404

If you think I?m full of it (I would be doubting this too) go over to the Roderus Custom forums and look in one of the 50BMG construction threads (don?t remember which one) and check out how to do the numbers yourself. I don?t want to come across in the wrong way, I?m not trying to piss on anybody?s parade, I?m just trying to share what I have learned from others, so that we can be safe, I like my face the way it is. The point I am trying to make is that max psi in a cartridge does not determine if it is safe to shoot in a specific action, bolt thrust does, and that psi and CUP are not the same thing. If you still believe that pressure is the sole determining factor of what and action can take, consider this. The max pressure of a 22lr is 24,000psi while a 12ga is only 11,500psi; a 12ga action is a hell of a lot stronger than most 22lrs.
 

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#51 ·
Not disagreing with you (still thinking) so what you are saying is that a8mm mauser(6069 thrust) puts less thrust on the the lugs than a 7.62x39 (6777 thrust)? you show a 308 and a 30-06 the same even though one has a larger case capicty for example the30-06 uses 4 grains more powder (vargert) than a 308 with the same bullet. at the same cup pressures. If the above calculations were true a 22-308, 308, 338-08, 458 win (same parrent case) loaded with different bullet weights to the same Cup pressures would all produce the same bolt thrust? you havent rained on any ones parade in fact just the opposite. It means that one could load the 8mm up some. I'm still taking this in and I do not doubt the math .good info. The $64,000 question still is what is a safe thrust pressure for a AK (Yugo) bolt and trunion. My caculator is on fire right now thinking of a boltface as a HYD piston does make sense. the 458 SOCM with 50,000 CUP must produce huge thrust numbers ( just saw a pic that was posted on the CAL that may work forum today of a AK chamberd for 458 SOCOM)
 
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