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Thread: Blowback Head Space x25 Conversions

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    Indian Admin Winn R's Avatar
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    Default Blowback Head Space x25 Conversions

    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?
    There is no nonsense so errant that it cannot be made the creed of the vast majority by adequate governmental action. -- Bertrand Russell


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  2. #2
    TRX
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    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.

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    aka: SDK1968 dutigaf's Avatar
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    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
    say what you mean & mean what you say


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    No Hope For Me 1biggun's Avatar
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    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.

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    Indian Admin Winn R's Avatar
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    Quote Originally Posted by 1biggun View Post
    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

    Quote Originally Posted by dutigaf View Post
    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.

    Quote Originally Posted by dutigaf View Post
    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.



    Quote Originally Posted by TRX View Post

    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
    There is no nonsense so errant that it cannot be made the creed of the vast majority by adequate governmental action. -- Bertrand Russell


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    No Hope For Me 1biggun's Avatar
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    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?

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    Indian Admin Winn R's Avatar
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    The consensus is that this design splits cases.

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




    There is no nonsense so errant that it cannot be made the creed of the vast majority by adequate governmental action. -- Bertrand Russell


    "Never attribute to malice that which can be adequately explained by stupidity." Robert J. Hanlon

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    aka: SDK1968 dutigaf's Avatar
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    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
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    No Hope For Me 1biggun's Avatar
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    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

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    Indian Admin Winn R's Avatar
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    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.
    There is no nonsense so errant that it cannot be made the creed of the vast majority by adequate governmental action. -- Bertrand Russell


    "Never attribute to malice that which can be adequately explained by stupidity." Robert J. Hanlon

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