Broken bolt catch

[308kiwi]

So I managed to break the bolt catch on my Aero M5 last week, (Aero bolt catch that came with the lower), ordered a couple of DPMS 308 catches from Brownells which turned up in the mail this morning, fitted with a lot less for/aft float in the slot in the lower.

After reading some of the info on here I thought I'd measure my bolt over travel,  bolt face measured at .120" rear of the back face of the bolt catch when bottomed out in the buffer tube, so I machined up a .080 Delrin spacer to drop down the end of the buffer tube, now I have a .040" gap between he bolt face and the catch.

Buffer tube is a AR15 carbine, Buffer is a stainless steel unit I machined up with a couple of tungsten ar15  weights in it, 6.5 oz and 2.530" long, as for the spring it is a LBE 308 Rifle spring that I shortened to 27 coils, wire dia is .070"

Any other suggestions from you guru's on what else I should look for as to the cause of my original failure.

 

Edited April 21, 2018 by 308kiwi

[Rsquared]

Sounds like you're all over it brother.

[308kiwi]

I hope so, was thinking of an adjustable gas block to tame it down a little more but really don't want to be trying to fix something that doesn't need fixed or can be fixed in a different manner, eg spacing the buffer and spring slightly further forward.

There are so few of these, 308 AR's, over here that the knowledge base for issues is equally tiny, hence why I am here, you guys have been around these things a long time and know your poop.

[98Z5V]

Bolt over travel is what did it, and why the internal depth of the receiver extension is as important as I keep harping on. 7" internal depth on the DPMS-based carbine recoil system isn't a "minimum depth" or "starting depth," as XXL Fitness recently, incorrectly, stated...

Lots of guys assemble pistol-caliber carbines with blowback operation. It all goes together with regular AR15 carbine recoil parts...  BUT.  The PCC bolt carriers don't have a collapsing, rotating bolt - it's fixed into the front of the carrier.  It's shorter, by a decent amount.  The choices are:  longer PCC-specific buffer, usually marketed as "9mm buffers," or stack a buck seventy-five in quarters into the receiver extension.  If one ignores those methods, broken bolt catches are the result, from bolt over travel.  Few PCC setups have a last-round hold open, so you don't see it as often - but those with bolt hold open on the last round will certainly break the bolt catch, in short order, when the advice is ignored.

Since I brought up PCCs - use some type of non-rotating pin system on blowback operated PCCs. If you don't, you will break hammer pins. It's not "if,", it's definitely a "when," and it won't take a high round count to do it. Blowback operation is hard on hammer pins.

My $0.02.  I think you fixed it.

Edited April 21, 2018 by 98Z5V

[308kiwi]

Thanks 98Z5V, it's your posts that have hammered this home to me

Yes, I've seen the same with PCC's, have seen several 9mm PCC's over here that have broken hammers through the narrow section of the hammer, I put it down to the same, very high initial bolt speed. Same happened to a .223 AR I saw once, that had a Ceiner 22lr kit fitted to it, broke a RRA 2 stage match hammer neatly in half after about 3K rounds.

Ran 100rnds through my M5 today, locked open at each mag empty, even with only .040" of over travel, which sorta surprised me, I wasn't sure that I had left enough travel in it.

 

Edited April 21, 2018 by 308kiwi

[98Z5V]

2 minutes ago, 308kiwi said:

Thanks 98Z5V, it's your posts that have hammered this home to me

Yes, I've seen the same with PCC's, have seen several 9mm PCC's over here that have broken hammers through the narrow section of the hammer, I put it down to the same, very high initial bolt speed. Same happened to a .223 AR I saw once, that had a Ceiner 22lr kit fitted to it, broke a RRA 2 stage match hammer neatly in half after about 3K rounds.

True, very true. You gotta run at least an H2 in a PCC, H3 being better. Heavier than that is even better.  I've survived thus far with the H3 buffers in mine. 

I've kinda turned into the "recoil system jackass bastard" on this stuff...  

[dpete]

And we appreciate all the "recoil system jackass bastard" guru advice that you dish out.     No bashing of the parts used, just "do this to get it fixed".     

This is the best "troubleshoot my system" forum I'm on.  No matter what the system.

The one part I tossed from my Aero 308 parts kit was the bolt catch.  The Phase 5 fits Aero reciever sets perfectly and works great.  If you like the BAD lever concept.

 

[Floydieguns]

Where did one produce specifications as to the maximum over-travel beyond bolt-hold open?

To start.. clarification on this is as follows:

Measurement taken as such: When bringing bolt carrier group/assembly rearward as in recoil/extraction stroke to full rear travel. Total gap between the bolt face and bolt catch abutment face.

If above is correct and I am to understand this..

Having more than a 0.040" tolerance will cause bolt catch to break?

Was under the impression that in-so-far that the BCG was not impacting the receiver&extension due to overtravel.. that any amount of space would be acceptable between bolt face and bolt catch via measurement as stated above.

That the root cause of breaking the bolt catch was typically caused by too much energy when the hardened bolt drove into a typically more low grade piece. Mine mostly appear to be cast, using dpms. However those fine seems may be machining chatter.

Which, with the above mentioned about 'energy' of bolt coming forward.. the original plastic buffer of early 1960's colt m16/civy single action sporters was replaced with a weighted buffer.

Knowing that armalite/fairchild had their design with a two-piece buffer of alloy...

Aftermarket began offering multiple action spring weights and 'more heavy' buffers. Also, adjusting/tuning the gas system came in to play.

 

*******

I simply want to understand which specification you all are using for this? I was unaware of such things. That was typically tossed up as fatigued parts, not enough buffer mass, old action springs..

Now.. all this in;

I agree that a longer space between the bolt face and bolt catch when measured as described at top of my comment, in math that extra distance can equate to a more large amount of energy being imparted upon bolt face abutment to bolt catch.

Though - is the receiver showing any visual signs of deformation from said catch being hammered very hard? One may sway the other side of this that less distance between the bolt face and catch during full rear travel could cause with weak magazine springs or an over-gassed/under-weighted action a lack of duration for bolt catch to come up completely.

I have seen a bolt catch grab the carrier and not the bolt face.

I have also had lemon parts before...

 

Not trying to trigger the snowflakes. Just - where did this number come from? It is like Schuemann barrels saying it is a specification of .015" tolerance between the slide and barrel vertically when the barrel is in bed and slide travels rearward during recoil/extraction. Browning never specified this. Colt never amended this. Schuemann spat this out and some people claim it is valid.

[jtallen83]

I don't think he said it was a specification. There are no overall specs for these rifles, maybe some manufactures have established a spec for their rifles but can't say I have seen this info in public. I try to get as close as I can and not have function issues. Gotta take into account that the buffer cushion is going to allow a little more travel on recoil and this will vary with the material used. It would be hard to say there is any single optimum spec.

[Floydieguns]

47 minutes ago, jtallen83 said:

I don't think he said it was a specification. There are no overall specs for these rifles, maybe some manufactures have established a spec for their rifles but can't say I have seen this info in public. I try to get as close as I can and not have function issues. Gotta take into account that the buffer cushion is going to allow a little more travel on recoil and this will vary with the material used. It would be hard to say there is any single optimum spec.

Yup. Am well aware rubber will compress and add to the rebound.

I was told a minimum of .040 and no more than .125". This was during discussion with a fellow armorer whom worked as QA in engineering with a manufacturer. 0.125" being excessive and may cause problems with bcg impacting the receiver extension or charging handle.

Was informed .060 to .080 would be more ideal. Anything more than .100" was un-needed. However, he amended that "if it is running fine, with no impact damage anywhere.. i would shoot it. If the rifle is in spec with headspace and is not having unintended striking on the receiver, gas key, charge handle or odd wear inside. Otherwise, it is good."

That was years ago.

He also said, "you will be replacing bolt stops. They break. Quite often. I have one rifle we use at the shop. It has been through ten barrels and countless small parts."

This was when I was just starting. Fellow claimed they had fired over one million rounds in that specific receiver set. Who knows.

A world class gunsmith for the 1911 and other handguns told me the same thing about barrel specs. "Is the lockup exact? Is the chamber good? Are you having odd wear? No? Then what the fornicate is wrong with it? You think too much. Stop thinking. Just do. If it runs with no malfunctions, no odd wear, it shoots true. Man, parts break. Slide stops fail. Hell, kimber MIM thumb safeties snap from using them. It is common. Do extractors last forever? It broke - fix it. You know how." This was during a conversation about why Schuemann claims a floating spec beween the barrel and slide during unlock. It was resolved as this - do not worry about it. Are you having that problem? I responded with 'uhx what problem? I do not understand why they claim a need for that tolerance. It is not on the original J.M.Browning blueprint. "EXACTLY!"

 

******

The end of my two posts is this;

Is there actually any need to control this measurement? If you are having no unintended wear/impact and the firearm is functioning.

[308kiwi]

The numbers I talked were from measurements made, the .120" over travel I had was from nothing else but a combination of the internal length of the buffer tube vs the length of the buffer and bolt group, I made the buffer myself, it's OA length was determined from what I had read on here and other searches around the net, consensus is that the DPMS 308 carbine buffer is 2.500" long, (mine is 2.530" long as the rubber squishy bit is .030 longer than I allowed for), had I made it 2.610 long then I would have had my .040" over travel that I have now, making a spacer up took 5 minutes, making a whole new buffer would take an hour or so, simple choice was made.

How did I come up with the thickness of the spacer I made?, .080", trust me, nothing any more technical than plucking a number out of the air, .120" divides easily by 3, I thought I may have to make several different thickness spacers to try as I was unsure that I would have enough 'dwell' time with the BCG at full rear and for the mag follower to react on the catch and have time to place the catch in front of the bolt face, turns out my first guess works just fine.

One of the most important lessons I have learnt since getting into the 308 AR 's is what the guys on here keep saying, "there aint nothing standard".

 

 

Edited April 21, 2018 by 308kiwi

[308kiwi]

4 minutes ago, 308kiwi said:

 

 

Edited April 21, 2018 by 308kiwi

[98Z5V]

29 minutes ago, 308kiwi said:

The numbers I talked were from measurements made, the .120" over travel I had was from nothing else but a combination of the internal length of the buffer tube vs the length of the buffer and bolt group, I made the buffer myself, it's OA length was determined from what I had read on here and other searches around the net, consensus is that the DPMS 308 carbine buffer is 2.500" long, (mine is 2.530" long as the rubber squishy bit is .030 longer than I allowed for), had I made it 2.610 long then I would have had my .040" over travel that I have now, making a spacer up took 5 minutes, making a whole new buffer would take an hour or so, simple choice was made.

How did I come up with the thickness of the spacer I made?, .080", trust me, nothing any more technical than plucking a number out of the air, .120" divides easily by 3, I thought I may have to make several different thickness spacers to try as I was unsure that I would have enough 'dwell' time with the BCG at full rear and for the mag follower to react on the catch and have time to place the catch in front of the bolt face, turns out my first guess works just fine.

One of the most important lessons I have learnt since getting into the 308 AR 's is what the guys on here keep saying, "there aint nothing standard".

 

 

Exactly, brother - I think you solved the issue.  Floydie is thinking WAY too far into this, and he saw numbers, and thought someone was posting "specs," - when there aren't specs on this platform. 

There are work-arounds - because there are no specs.

When I built my first 308AR, it was all DPMS genuine guts, with Fulton Armory BCG and barrel.  I didn't measure anything, but headspace, and put it together.  The second one I build was all mixed parts on Aero receivers - and I got a weird carbine receiver extension.  And I had weird function issues that I couldn't trace to anything that I knew about these things.  I ripped the whole gun apart, and started measuring everything out against that first DPMS rifle, and the damn receiver extension was 7.100" internal depth, and the spring didn't match anything for "proper references" on 308 carbine springs, and the 2.500" buffer was WAY too light to be correct.  That first DPMS was a rifle recoil system, and I had to measure out my next 308AR carbine extension against real, true AR15 quality carbine extensions.  I looked at everything I could think to look at. Initially, I was ready to think that the AP receivers sucked - and that's definitely not the case, no, not at all...

The whole recoil system went in the trashcan, and I picked up the complete (pieced together) Armalite AR-10 Carbine recoil system.  And the weird function issues went away, immediately.

It was really at that point right there, that I got into the recoil systems on these things.  I didn't know shiit about the differences until it happened to me.  Since then, I've tried to learn as much as I possibly can on them. 

[98Z5V]

One thing Floydie said that I don't agree with...

He also said, "you will be replacing bolt stops. They break. Quite often. I have one rifle we use at the shop. It has been through ten barrels and countless small parts."

Nope. Not true.  I've got a handful of these Big ARs now, and I've never broken one of the bolt stops.  They certainly don't break bolt stops, quite often.  He was given bad information from someone that didn't determine the cause of his own broken bolt stops, and fix it properly.

Also, in reference to this comment from him:

A world class gunsmith for the 1911 and other handguns told me...

My $0.02 on that...  A "world class gunsmith for the 1911 and other handguns" is not necessarily smart on AR gunsmithing.  A world class 1911 gunsmith qualification does not automatically give you a "world class" qualification in everything, every platform... 

[308kiwi]

36 minutes ago, 98Z5V said:

Also, in reference to this comment from him:

 

My $0.02 on that...  A "world class gunsmith for the 1911 and other handguns" is not necessarily smart on AR gunsmithing.  A world class 1911 gunsmith qualification does not automatically give you a "world class" qualification in everything, every platform... 

So true, I've been around for a while, have barrelled countless bolt rifles, some of which have won national and international long range titles, does this make me "a world class gunsmith", pfffft, not even close & I aint that vain.

My day job is a Heavy Diesel Mechanic for a major brand of tractors and combine harvesters, been doing it for 35 years, still learn new things on a daily basis and one thing I constantly hammer home to our apprentices as they come through is don't ever think you know all there is to know cause it'll bite you right on the ass.

[98Z5V]

13 minutes ago, 308kiwi said:

My day job is a Heavy Diesel Mechanic for a major brand of tractors and combine harvesters, been doing it for 35 years,...

Ironically, my day job now is high performance race engines for powersports products...     Getting 230+ hp to the tires, through a CVT transmission, from a one liter engine is a rush...  

You measure everything, you tech everything, you check everything, and you clearance everything.  The minute you don't do that,  you blow up something that ends up costing you alot of money...

Edited April 22, 2018 by 98Z5V

[Sisco]

20 minutes ago, 308kiwi said:

So true, I've been around for a while, have barrelled countless bolt rifles, some of which have won national and international long range titles, does this make me "a world class gunsmith", pfffft, not even close & I aint that vain.

My day job is a Heavy Diesel Mechanic for a major brand of tractors and combine harvesters, been doing it for 35 years, still learn new things on a daily basis and one thing I constantly hammer home to our apprentices as they come through is don't ever think you know all there is to know cause it'll bite you right on the ass.

Wisdom