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a stronger AR-bolt-is it possible?

4K views 36 replies 15 participants last post by  j427x 
#1 ·
during my last tour of that dirty-dusty sand-flea infested unmentionable place. i did witness a couple of m4 bolts that failed. now even the mention of this fact will get you banned from some of the mall ninja sites. i don't know if this was some kind of QC issue with that batch of m4's or just the general abuse and bad conditions adding up to the bolts breaking the lugs off in the field.

i sort of think the cause was very long periods of sustained full automatic firing, being too dry and getting some of that flour like sand in the barrel extension--but that is just my theory.

anyway with the advent of 6.8spc, the 6.5grendel and the return of the 7.62x39 to the ar-15 type of platform. there has been a increase in bolt failures. nothing drastic--some will pretend it could blow the bolt any second. but enough for concern as the little 5.56 bolt has been streached far beyond its origional design.

the 6.5 looks to be the best candidate for a failure--with the bolt really opened up and AA's bolts have the additional "feature" of having the bolt face recessed an extra .010?? mine hasent broke yet. and i don't think it will any time soon no more than i fire it. but i wouldn't want to trust my life in the sand pit to such a thin bolt and i am thinking if you really put the rounds through it- that bolt is a likely suspect for a failure. even if it is made of kryptonite!!

7.62x39 bolts are about the same--but generally arn't cut as deep. you see on the internet more 7.62x39 bolts that have failed. some think these failed bolts came from china--but i suspect that the 7.62x39 bolts fail because 7.62x39 is still cheap compared to 6.5/6.8 and there for get a lot more use and abuse.

OK now we have settled the WHY-- lets look for a solution.

if you wanted to put a hot round in an AR type rifle-- could there be a way to strengthen the bolt without increasing its size too much? the idea is to get as much as you can without going to a bigger platform like the AR-10

i have often thought a six-lug bolt with slightly larger and thicker lugs "might" solve the problem. you would have to have a coustom "headspace" and re work the barrel extension for such an attempt.

or maybe just lugs that are a bit longer--and reach back further on the bolt maybe??


then the limit would be the barrel extension. but i have never seen a BE fail?

any thoughts on the ar bolt??
 
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#2 ·
I think it was stoner or someone working with the stoner 63 rifle that came up with making the lugs on either side of the extractor a little thicker to compensate for the missing lug where the extractor is in the ar15 type bolt. I don't think you could change the bolt design much without also changing the bolt carrier which controls how much the bolt rotates. It would seem to me that if you went to a fewer number of larger lugs then you would also have to change how much the bolt needs to rotate in order to turn into and out of battery. I also think a bolt with 5-6 bigger lugs would strengthen the design and might also help it run better when its dirty.
 
#3 ·
If you have a round with a smaller diameter case head you'll have less bolt thrust, though you can't really make a longer case fit in the AR...

Maybe try lapping your bolt lugs to the barrel extension, that would reduce the amount the bolt flexes in use, and increase it's life. You just gotta find a bolt that headspaces tight...
 
#4 ·
some broken 7.62x39 bolts

notice the broken bolts broke at the same place.

one wonders if this weapon has some kind of out of spec barrel extension?

would an reinforcement of that one broken lug shore the 7.62x39 bolts up?

on my latest 7.62x39 CAR i did cut down the lug opposite the extractor by 2/3ds height. sort of like the LMT bolt. this bolt is a model 1 sales bolt.
 

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#7 ·
#6 ·
The bottom line is the AR in itself is a week design and that is why we are the only country to adopt this system. I have several and enjoy shooting them but lets face fact there are better wheels out there. If the bureaucrats in DC and the DOD ever got there moral compass in the correct direction our troops would be equipped with the best. Now that the F22 is scrapped maybe our guys can get up armoe and some cleaning kits. Sorry Just spoke to a couple of friends from the sandbox and I am sending them cleaning supplies and replacement parts.
 
#13 ·
W.T.F..

The bottom line is the AR in itself is a week design and that is why we are the only country to adopt this system. I have several and enjoy shooting them but lets face fact there are better wheels out there. If the bureaucrats in DC and the DOD ever got there moral compass in the correct direction our troops would be equipped with the best. Now that the F22 is scrapped maybe our guys can get up armoe and some cleaning kits. Sorry Just spoke to a couple of friends from the sandbox and I am sending them cleaning supplies and replacement parts.[/quote

Sorry to bust into your thread j4. But why the hell are you having too send cleaning supplies to our heroes in the sand?????
Mother Friggin Haliburton is charging the Guberment $95 for a load of laundry and those guys cant get cleaning equipment. What the F&*(K am I reading.
Tanvil had a thread going on sending scopes to guys over there. WTF else are we not hearing. :frown:
 
#8 ·
yea the extension is slightly biggger as is the bolt. I doubt the weight increase would be much. remington gives a weight of there gun it could be compared to a AR with the same profile barrel to get a general idea of the increase.

The bottom line is the AR in itself is a week design and that is why we are the only country to adopt this system.

WRONG
lots of countries over 80 that have used the m16.
M16 rifle - Wikipedia, the free encyclopedia

Imoo the bolt streanth of the AR is the least of the guns design problems. Id be more concerned about the jamming when dirty and other operation issues.

while not as strong as the AK bolt the AR is not really that bad. if you compare some issues other arms have had thoughout history.

It may be me, but seems that most bolt failures are in commercial AR15 that are possably not up to the specs they should be. there is also issues from non 5.56 nato ammo and calibur that the bolt was never intended for. Im no expert here on AR,s/M16,s
and the above pics show more pics of broken bolts than i have even heard of failing in a AK so obviously there not perfect.
 
#9 ·
I can see how the 762x39 bolts would be more prone to cracking at the bolt lugs. The ring that encompasses the case head is much thinner. Once a crack started there it could continue until the lug failed. All of my ar15 bolts are the 5.56 size and so far I haven't had any issues with cracking. I wonder if those broken bolts were out of a car length gas system since they open at a slightly higher pressure than the 20" length?
 
#10 ·
i have seen m-16's with 50,000+ rounds on the bolts with no bolt failures and carbines/M4s with 10,000+ of 5.56. the strength of a good bolt in 5.56 is not a question.

the life of the bolts is pretty darned good--considering no defects or bad steel when they are made. lots of nations use the m-16 series and lots of military weapons besides the m-16 uses the same or at least very similar type of bolt.

what this discussion is about is the larger cut bolts for the 7.62x39/6.5g rounds as it has a thin section near the extractor cut out.

back to the discussion on how to strengthen the 7.62x39 bolt--

if the extractor was made a bit thinner--giving a little more meat for the lugs to hold to the bolt? would the extractor still be functional???
 

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#11 ·
the strength of a good bolt in 5.56 is not a question.
Agreed. Some designs are stronger but the AR bolt is stong enough.
if the extractor was made a bit thinner--giving a little more meat for the lugs to hold to the bolt? would the extractor still be functional???
I believe so and here's why. The 7.62x39 is a tapered cartridge and therefore requires much less force to extract vs. a "tube" style cartridge that inherently has more friction such as the 5.56 design. Thus, a smaller extractor on an AR style bolt makes sense to me. Or a fluted chamber like the Germans did with the G3.

I don't know how small it could go, but couldn't you also have an extractor that straddles a lug, providing 2 exractor fingers touching the base? Just thinking aloud.
 
#17 ·
Before everyone started putting verticle grips on the front, most bolts in 5.56 broke at the cam pin hole. Both of my high milage ones did as well as both my chromed bolts(improper post plating treatment results in embrittlment). You can actually pull back on the front grip hard enough to pull the barrel slightly down as the upper can have slightly loose barrel fit and this area of the receiver is weak so it can flex. Hanging all kinds of accessories off the front of the gun make this worse. This all causes uneven lug engagement. The top lugs break as a result.

-yarro
 
#18 ·
j427x, I've been researching a fast ar15 6mm round for a shits and giggles build and was shown a newly esigned bolt for the ar15. It's from the same company tony549 linked to, and has been added in the past two years. 6MMBR Bolt & Extension for AR15 It's claiming to hold up to a 6mm 105gr bullet at 3000+fps with a .473" case head. It's pretty close to what remington says is unsafe for it's 30ar and the reason they went with a ar10 sized bolt on it. I'm leaning towards a 6mm dasher upper using this bolt and barrel extension put on a remington 700 barrel.
 
#19 ·
j427x, I've been researching a fast ar15 6mm round for a shits and giggles build and was shown a newly esigned bolt for the ar15. It's from the same company tony549 linked to, and has been added in the past two years. 6MMBR Bolt & Extension for AR15 It's claiming to hold up to a 6mm 105gr bullet at 3000+fps with a .473" case head. It's pretty close to what remington says is unsafe for it's 30ar and the reason they went with a ar10 sized bolt on it. I'm leaning towards a 6mm dasher upper using this bolt and barrel extension put on a remington 700 barrel.
yea this is interesting.

looks like it would be THE set up for that military round everybody keeps dreaming up. and it keeps everything in a compact size required for it to stay in the little AR!:headbang:
 
#20 ·
Looking through their website it looks like they offer it in two sizes. The .473" size and a smaller one for x39 size IIRC. There is also a 6mm turbo 40 that is based off of the ppc that is close in velocity for what I'm looking for (105gr at 2850fps). I have found the drawings for a dasher reamer (its basically an improved 6mm bench rest) so I'm leaning towards that right now.
 
#21 ·
I am an FFL /SOT and shoot a LOT of FA in M16s. Mostly Wolf ammo to boot. None of my guns, to date, has cracked a bolt. Have cracked one cam pin though.

I am anal about inspecting the bolt and cam pin every shooting day and suggest others do so at LEAST as often.

Obviously, i do not shoot in the miserable conditions that our folks in the sandbox do. Not sure if that has any effect on the bolts but it sure wears everything it gets in (and it gets in EVERYTHING) so can't be good for the bolts either.

I am no engineer but am an A&P dealing with highly stressed parts. IMHO the bolt issue is less one of design than one of quality. I would NEVER use a plated bolt due to hydrogen embrittlement. I would never use a bolt from a questionable source. The type and QUALITY of the steel and in particular the quality of machining a bolt has a HUGE effect on it's strength! Tiny tool marks and / or poor radi can lead to stress risers and cracks. Very highly stressed parts that are "on the edge" in aircraft are beautifully finished, usually polished with generous radius at areas like the base of a locking lug. There is a reason for that - and an associated cost!

I have looked at a lot of bolts at gunshows and in customer's rifles. Some of them are VERY poorly finished and likely to crack eventually, IMHO. I suggest you inspect YOUR bolt and a quality one, such as Colt and decide for yourself if your's is "good enough".

Admittedly a better design, such as the AK is less susceptable to these kinds of problems but we are pretty much stuck with what we have. Until a better design (which would be of the bolt AND barrel extension most likely) we need to deal with it by bolt QUALITY.

just my .02 worth
 
#23 ·
yea i haven't broke any 7.62x39 bolt lugs "YET" but i know of plenty of others that have.

i cut down that lug on one of my 7.62x39 bolts like armalite did on their 5.56 bolts --so far it ain't broke.

the problem is when you open up the ar bolt big enough for the 7.62x39 rim there ain't much meet left for the bolts lugs to hold to the bolt base.

now that 6mm-ppc bolt is a step in the right direction!
 
#25 ·
Nah..

Cant be China. Illegal too import gun parts from China.
Holy mother of God.
I read through this whole friggen thread and now I am afraid too shoot these damn things. I should have stuck with my AKs.
Just spent an hour getting a fish hook out of my finger. Now all I need is a bolt in my eye ball.
 
#27 ·
Cant be China. Illegal too import gun parts from China.
Holy mother of God.
I read through this whole friggen thread and now I am afraid too shoot these damn things. I should have stuck with my AKs.
Just spent an hour getting a fish hook out of my finger. Now all I need is a bolt in my eye ball.
don't worry 4th! were talking about 7.62x39 bolts and overpowered wild cat things like 6mmBR.

the 7.62x39 bolts don't break all that regularly and there the thinnest of the bunch.

one of my 7.62x39 uppers has a couple thousand down the tube so far so good.

i sort of think the spat of bolts breaking in ar-15 and even in some AKMs may be due to just a batch of rotten steel that has been floating around for the last few years.

even though the bolt can't come from china it could be made from china sourced steel!:confused:
 
#28 ·
Chinese marked parts would be illegal to import, unmarked ones trans-shipped through another country with "Made in Taiwan" or similar on the crates would get in. Besides, the PLA pretty much owns a large chunk of the Port of Long Beach, ya wanna bet their "foreign trade zone" warehouses have more than a few naughty things in them that eventually wind up here outside of "normal channels"?
 
#29 ·
What yaro said... and as for Mod1 bolts breaking they are poor quality no mater where they are made.. you usually get what you pay for.. I've always seen M16 bolts break at the cam pin with the old wolf ammo that was very common.. cause... lugs broke more rarely in the Military.. the M-4 was after my time..so I can't say about them.. but they need a bit more gas to function that a longer barrel gun.. Also.. I believe the MP inspection kept a higher quality parts in Military guns and the rejects went to the gun shows.. B2B
 
#33 ·
Thanks BBill, I sent them a email as the 6mmbr super bolt and extension with the round lugs has disappeared from their website since sunday. I hope they're just out at the moment.
 
#35 ·
I had an second run ARP bolt in 6.8 years ago. It is thicker compared to the Busmaster (CMT?) bolt that came with the rifle. I didn't shoot it much with the ARP bolt and I wasn't too impressed with it because it used a 5.56 extractor and a modified extractor spring. I didn't have any failures wiyh it on an intermediate gas system Bison Armory barrel. Although I didn't have any failures with the 1st run Bushmaster 6.8 carbine it was originally purchased as.

Anyway fast forward to 2011. I built a 7.62x39 AR15 upper with an Osprey Defense piston kit. The Bolt is from SI Defense and supposedly MPI, carpenter steel and the whole milspec deal. I also run an AA bolt spring and POF roller cam pin on the OPS one piece bolt carrier. I hand cycled it dry for a long time while locating other patrs needed (including the POF cam pin and bolt spring) and had some where on the back of the lugs and cam pin hole. I never shot it DI but soon after installing everything it hand cycled much smoother and as far as I can tell no more wear (parkerizing rubbing off) on the bolt when lubed and fired.

If I was going to entertain the thought of this carbine as being a serious fighting one I would probably get the ARP bolt for piece of mind. But as a range toy and occasional deer rifle the SID bolt is good enough. I am just shooting steel cased Wolf through it and passing a bore snake through every 100 rounds until it jams. So far the bolt seems to be holding up and no signs of hard extraction.

YMMV
 
#36 ·
Well, for what it's worth. I have sent several emails to AR15 Performance over the last month+ without any response asking if they are still making the 6MMBR Bolt & Extension since it is no longer shown on their web site. Since the project is basically stopped without that bolt and barrel extension I'll most lilkely go another direction.
 
#37 ·
i think were going to see a lot of the makers of one-off stuff for AR-15 type rifles start dropping off the map due to the poor economy and slow sales.

hell i found a place that will make barrels to order but the prices on ammo make if difficult to run off a batch of 6.5g type barrels. that other ar place is sitting on a stack of 6.8s and their moving very slow i have been told.
 
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