Underground Tactical bolt failed

Talk about the AR15 style rifles chambered in 450 Bushmaster.

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Underground Tactical bolt failed

Postby Bmt85 » Mon May 07, 2018 6:22 pm

Well I was doing load work up in my 20” BHW to see how it matched up to my Tromix and Bushmaster uppers. I was loading 275 TSX with Lilgun. First round was 36gr and 37gr, everything looked great, so I moved on. Went to 38gr and 39gr. Fired the 38gr loads and they looked great, nothing wrong, so I fired the 39gr and the second shot locked up the gun. I checked the first 39gr that was fired and it had heavy imprinting. Got home and mortared the rifle. Second 39gr brass looked worse then the first! First thing I did was pull the BCG, and sure enough, a lug cracked, and was slightly bent in. I checked my measurements on the 38gr loads, and they tracked right inline with what I was expecting to see. The 39gr charges had EXCESSIVE imprinting and case head growth. All brass from 36-39gr was once fired Hornady and all primers looked good, the primers from the 39gr charges were a little flattened, but nothing crazy. I think that had more to do with the bolt failure.

What I’m gathering here, is that the Underground Tactical bolt probably wasn’t heat treated correctly. This isn’t the first bolt from them I’ve seen have fail. Heard of a couple Grendel guys, too. This bolt only had about 100-125 rounds on it, most of which were factory ammo or handloads well within book, not showing even the closest sign of over pressure. So, I’m going to swap it out for my Tromix bolt, which has around 3 times as many firings, with a good portion of those pushing to find max loads, and still holding strong. Then I will start over and work up again, and see what happens. Posted a pic, hopefully you can make out the crack.

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Re: Underground Tactical bolt failed

Postby Hoot » Mon May 07, 2018 8:37 pm

My experience with bolt imprinting, not to be confused with swiping, has been that despite trying to cling to the chamber walls, the brass backs out into the bolt face due to pressure. Hypothetically, it can also be the result of the chamber getting contaminated with excess lubricant flying around but I only experienced it on occasions where I was at the top of a set of charge steps, in some experiment, on the threshold of too stiff and usually with powders that are slower to reach full pressure than Lil Gun. Still, conditions such as a hot barrel/chamber and a round sitting in there long enough to soak up the heat, can make for more pressure than normally encountered. Ball powders are sensitive to temperature. Just throwing some ideas out there.

I've mentioned this about a half dozen times but their isn't too many for this: If you like to play near the top, ball powder loads that were previously developed in cooler conditions, can step over the top once warmer weather returns, especially for shooters who chamber a round and think on it for a while before breaking the shot. While there are few things more exasperating than waiting for a barrel to cool down, it can be made more bearable by bringing a second or third weapon to the range to switch to experimenting with every so often. That or make a barrel cooler out of a cheap ($20) 12V car vacuum and a gel cell. Shove it over the end of the barrel and with the action open, give it a minute or two run time. Its impressive what a breeze can accomplish in short order.

That having been said, I've never had imprinted cases cause a bolt failure in my AR's. I'd send the bolt back to the manufacturer for a follow up analysis (in their interest).

Hoot
In Theory, there is no difference between theory and practice. In Practice, there is.
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Re: Underground Tactical bolt failed

Postby plant_one » Mon May 07, 2018 9:49 pm

i'll second with hoot on sending that bolt back in too.


also make sure you check your barrel extension. any round that went off with enough pressure to damage a bolt like that could have done other stuff to your rifle as well. better safe than sorry!
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Re: Underground Tactical bolt failed

Postby Bmt85 » Tue May 08, 2018 1:37 pm

Well everything was done the same as any other time I do load work up for the 450B. I've dealt with imprinting before, but never had it happen that bad, that quickly, that's why I'm thinking the bolt failed during the firing of the 39gr loads, and caused the pressure signs.

I'll have to see if I can get a hold of them. I'll also pull the barrel and check for any issues.
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Re: Underground Tactical bolt failed

Postby abingham » Fri May 11, 2018 9:37 pm

That's a little disheartening. I have one of their lefty bolts in the gun safe waiting on my 450 build to come together.
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