Feeding problems

Talk about the AR15 style rifles chambered in 450 Bushmaster.

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Feeding problems

Postby Linhall450 » Mon Apr 15, 2019 9:16 pm

I have 16” carbine. It will cycle by hand but not when fired. I have 2 bushmaster metal mags and I have a Lancer mag with a Tromix follower. I built the gun with a heavy buffer and spring and when it wouldn’t feed I switched to a standard buffer and spring,same result. Anybody else have this problem?
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Re: Feeding problems

Postby Buckeye45 » Tue Apr 16, 2019 10:18 am

Check to make sure gas block is aligned properly or the gas tube is installed correctly. If it cycles by hand then you know mechanically it’s correct. No response when fired with the standard tells me your not getting enough gas to function. I know this from experience unfortunately.
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Re: Feeding problems

Postby 303hunter » Tue Apr 16, 2019 7:22 pm

Is your barrel dimpled for the set screw on the gas block? There should be small gap between the back of the gas block and shoulder. About the thickness of a credit card.
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Re: Feeding problems

Postby plant_one » Sun Apr 21, 2019 9:32 am

we're gonna need some more info likely. please give as much as you can.

a couple things that may be helpful

- did you build or buy the upper?
- who's (mfg) barrel is it
- adjustable stock or fixed (a2 style) stock
- what brand lower & did you build or buy
- does the lower function normal with another upper of a different caliber?
- what ammo are you using to test with






simple test for you on cycling to help further diagnose...

put a single round in the mag and fire it.

does it Bold hold open (BHO) for you?

if it doesnt, see if you can record it being fired. you may be able to tell if the bolt is cycling back far enough or not. If its not you know you have too little gas and need to look for a blockage or a leak, if it is its likely an overgas problem and you're getting bolt bounce and probably overrunning the follower

HTH
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Re: Feeding problems

Postby Mdl1911a1 » Mon Jul 08, 2019 11:00 pm

New forum member here, with the same question. Since this thread has been dry for 3 months, is it better protocol to hijack it, or to start a new thread with essentially the same question? Moderator: I can copy & paste to a new posting if you prefer.

some background: I have a 16" Bear Creek .450 Bushmaster barrel mounted in a Delta Tactical enlarged opening upper, on a palmetto lower. Stock is a full length Ace Double Star skeleton with stock tube, spring and buffer. Gun is a complete build, no pre-assembly except ejection cover and forward assist. Lower functions well with my .458 pre-built upper. I've so far used 1 box of Hornady 450 Bushmaster 250 gr FTX and 2 boxes of Winchester 250gr Deer Season in trying to work out feed problems. I have the Lancer mags with Tromix followers.

I've made the recommended mods per Tromix of opening up the feed lips, polishing the feed lips, and adding a front cut out. My 450 A-Zooms feed nicely from the mag to the chamber by hand; both 1st round and next enter the chamber, extract, and feed the next. Bolt holds open on empty mag.

I thought originally that the bolt was cycling too fast and not giving the round time to line up. But the use of either a Wolfe extra power spring, OR H2 weights in the buffer actually prevents the bolt from cycling at all. So I thought I should disassemble the front end to evaluate my gas block install. No carbon leaks were found around gas block or tube. Carbine-length tube was intact and not collapsed or blocked in any way. After pulling the 0.750 (correction: 0.875") diam gas block, the set screw and dimple had the block centered nicely on the barrel port as shown by carbon. (Barrel port is same diam as #50 drill shank; 0.0694".)

My on-hand lgs experts tell me failure to feed is ALWAYS a mag problem, so I have a Bushmaster brand mag on back order. (I suspect from the manual cycling with Lancer mags this isn't the problem, but hell. It's worth a shot.)

Oh. DTTs Upper has two little inlets for the feed ramps, but nothing for the center. I think the upper cross piece may be slightly blocking the barrel feed ramp. I tried posting photos of the feed ramp but the 120kb per picture limit didn't allow any clarity. I also tried videoing one shot. I caught the jam, but did not get the bolt movement.

ANY thoughts on what to try next? I am reluctant to open the gas port, but I do have an adjustable gas block also on order JIK. Also, any recommendations on slo mo apps for android?
Last edited by Mdl1911a1 on Tue Jul 09, 2019 7:25 pm, edited 1 time in total.
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Re: Feeding problems

Postby Hoot » Tue Jul 09, 2019 4:51 am

Welcome Aboard OM!
Not to worry. This thread is exactly where you should post this but also don't worry about creating a new thread if you feel like it going forward. Either way works and we don't charge by the thread. ;)

Before I make you think I can help, let me say that I don't own any polymer mags but is the lancer mag the same one you use with your 458s? Though the 450b is something like .060 narrower, I'd expect what works for one would work for the other. If the lancer mag is something you got specifically for your 450b, can you try it in your 458s, which sounds like it had a track record of running reliably.

This was mainly a useless, welcome aboard post. I'll butt out and let members with lancer experience wade in. Many issues with new builds find their way back to needing to get the rough surfaces broke in. Run it wet with a quality lubricant and clean the slurry off regularly before re-oiling. I realize that's almost as bad advice as 'Take two aspirins and call me in the morning" but its true.

Good Luck with getting it resolved.

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Re: Feeding problems

Postby PyroRobby » Tue Jul 09, 2019 10:17 am

My upper is a BCA 18" with mid length gas system, your 16" carbine is likely the shorter carbine length gas system. You will know this if the gas block is nearer the chamber than the middle of the barrel. Mid-length is middle of the barrel and rifle is closer to the end of the barrel.
Of the 4 different 450B mags I have, the best working mag IMO is the c-products 5 round, then the Long Load 7 round. The Stoner is too tight and scores the brass. The Lancer just doesn't hold the rounds well. I have a couple pro mags too and they didn't work well for me either, with either 450B pop-in followers or the tromix follower. All the mags I had to do mods to; but have the best results from the c-products.

The c-products slips in and out as it should and feeds perfectly now, had to notch out the front a little more than how it shipped. The Long Load is really tight in the mag well and I have to force it in and out; but it feeds to expectation. All the others just don't hold the rounds right, they all want to point the rounds up and not lay them flat.

Gassing issues I had, the port was the .089 dia spec from BCA but the rifle short cycled the bolt back so that last round would not lock the bolt open, didn't load the next round except manually also. Some mags would put too much pressure on the bottom of the bolt and not allow the bolt to cycle well also. Using the c-products 5 round mag or the Long Load 7 round, the feed works perfectly. Once I drilled out the gas port from a #43 drill (.089) to a #42 drill (.093) all under gassing issues and failing to lock back on last round ended. After solving the under-gassing, I also went up to a H3 buffer weight, because I had one and the bolt still blew back properly with it after porting. I really didn't notice a change in felt recoil with the H3. Biggest change in felt recoil was the added stock recoil pad and muzzle break.

Suggestions: Be sure your mag isn't loading too heavy a pressure against the spring tension and that the bolt doesn't have too much resistance to cycle because of pressure cause from the cartridges in the mag pressing up against it. This might happen from an overloaded new mag or a mag that just doesn't fit properly in the well. Be sure that the lips of the mag are not biting into the brass and cutting it due to improper fit of the lips pressing too tightly against the cartridge being fed, which would keep the rifle from feeding; but not keep it from locking open on last round fired.

Note: If you have fired at least 100 rounds through the new upper and it still fails to cycle; but you can cycle it manually, with no unusual resistance, and you don't have a gas leak, then you might consider porting your barrel a little. ( a little is .004 more than it is as the next increment in size). Also carbine gas systems tend to over-gas, so this would be odd if it is under-gassing and not due to leakage. Under-gassing can be at the gas block not aligned properly and restricting the port. It can be a leak at the tube and gas block. It can be a leak at the bolt carrier. If you have a standard buffer spring and standard buffer weight and don't think it is leaking anywhere and the bolt seems to cycle with the normal resistance when done manually, it would be odd for a carbine length gas system; but maybe it needs porting. The shorter the gas system, the more pressure impinged from the barrel, so under-gassing is more likely from a rifle length gas system or a leak or a restricted gas flow to the barrel port, misaligned, clogged ... etc.
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Re: Feeding problemst th

Postby Mdl1911a1 » Tue Jul 09, 2019 12:26 pm

HOOT; Thank you! The Lancer mags are modified just for the 450 Bushmaster. The 458 feeds just fine using a variety of govt and comml mags with standard 5.56 followers. I've tried Stoner AR .450 Bushmaster-specific mags also, but the modified Lancers work better. For the Stoner mag, the bolt fails to strip a new round, and rides across the top of the bullet before jamming.
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Re: Feeding problems

Postby Mdl1911a1 » Tue Jul 09, 2019 12:34 pm

PyroRobby: this gun has the carbine length tube. On last night's disassembly I was specifically looking for misalignment, leakage, or damaged components from poor assembly. But everything checks as good. This is a 16" Bear Creek barrel ported at the carbine length, and the port is 0.0695"; or a tight fit for a #50 drill shank. Should I contact BCA?
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Re: Feeding problems

Postby PyroRobby » Tue Jul 09, 2019 1:58 pm

Mdl1911a1 wrote:PyroRobby: this gun has the carbine length tube. On last night's disassembly I was specifically looking for misalignment, leakage, or damaged components from poor assembly. But everything checks as good. This is a 16" Bear Creek barrel ported at the carbine length, and the port is 0.0695"; or a tight fit for a #50 drill shank. Should I contact BCA?



If you think it is under gassing due to maybe needing a larger port size, then yes. BCA was very quick to get back with me when I had my issue, let me know it was covered by warranty and they would swap the upper or just let me send back the barrel. I told them I appreciated the support; but I was just going to resolve the issue myself, and I did. Drilling it will void the warranty, I didn't care, I wanted to fix it and not wait on shipping. Had a hunting trip scheduled.

A carbine gas system will have a smaller diameter gas port than a mid-length, and a mid-length gas port will be a smaller port size than a rifle length port. As the barrel length increases, along with the gas system length, the volume for the gas expansion increases, so the port needs to increase to have the mechanical pressure to work the bolt properly. With the increased volume, comes the need of a port size increase, due to a drop in pressure due to the greater volume containing the gas expansion. So, your port size of .0695 might be BCA spec. My port size I think is supposed to be BCA spec too at .089. However, from charts I have seen, googling AR15 gas port size chart, for me it would just be speculation. Somebody here likely can tell you more about what size port to expect than I can. Though my port was spec, nothing solved the short cycling until I shave .004" out of the hole, then it worked flawlessly. Of course if you plan to suppress the rifle then the port size might be fine with a suppressor.

You should have a 16" .750 diameter barrel and this chart says min is .070, I don't think it would hurt to go to .073, since the max for your call out is .086. I am not an expert though, just a nube really. Drilling the hole was easy, just remember to put a dowel in the barrel and take it slow and easy, if you go that way. BCA will warranty it though so, maybe just talk to them and do what feels right to you.
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