Incomplete cycling

Talk about the AR15 style rifles chambered in 450 Bushmaster.

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Incomplete cycling

Postby Quill » Sat Sep 01, 2018 3:06 pm

This spring I put together a new AR-15 in 450 Bushmaster. It is based upon an 80% lower with a JARD AR15 #4 trigger and a JSE upper with 18" barrel in 450 Bushmaster. The magazines are the 20rnd 223 version by Lancer. When factory ammo from Hornady is used everything works fine. However, when using reloaded ammo there are consistent failures to feed due to incomplete cycling. The bolt fails to pick up the next round and either closes on an empty chamber or hangs up on the top of the next round. The last round in the magazine ejects but the bolt does not lock open. I am using the Hornady brass and Hornady 225gr and 250gr bullets on top of IMR 4227 up to the maximum loads recommended by Lyman. I do not want to exceed recommendations for fear of damaging the rifle. What are the things that can be done to allow lighter loads to function while still safely handling factory ammo?
I have been reloading for close to fifty years but this is my first experience with semi-auto/AR-15 rifles and these kind of problems baffle me.
Thank you
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Re: Incomplete cycling

Postby Al in Mi » Sun Sep 02, 2018 9:49 am

Welcome.

Try different powder if you have some, 4227 a little slow.

Also check your gas port size on the barrel, might have to open it up a little.
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Re: Incomplete cycling

Postby giterdone » Sun Sep 02, 2018 2:19 pm

Your problem may be the buffer weight or buffer spring tension. When I built my 450BM 18" upper i started with a H3 buffer and experienced problems similar to yours. I replaced it with a lighter H2 buffer and the problem went away. You may have to use a lighter weight buffer to get it to cycle correctly with your weaker handloads. IMHO.... It appears to be under gassed with your reloads.
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Re: Incomplete cycling

Postby plant_one » Mon Sep 03, 2018 11:14 am

this could be several issues you're experiencing. we'll need more info to help determine your problem.



did you build the upper or buy it assembled?

how many rounds thru it?

carbine gas or midlength barrel? what MFG?

what BCG do you have? (ie: ar or m16 carrier)

what buffer (carbine, H1, H2, H3, rifle, etc)

are the lancer mags the ONLY mags you've tried?

can you give us more info on the loads that failed t0 cycle correctly? the more info the better - please be as specific as possible - bullet, primer, powder charge, crimp (if any), OAL, brass details



have you taken the gas block off to check for proper alignment?


4227 generally should produce enough gas to cycle an otherwise properly assembled and functional upper, especailly at or near max charges.
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Re: Incomplete cycling

Postby Quill » Fri Sep 07, 2018 4:29 pm

I will try reply's to my first post responses.

The upper is a JSE 18” 450 Bushmaster 1x24 Nitride w/ Young Rifle Customizable free float & Micro gas check. They replaced the rifle length forearm with a carbine length at my request. I am using Lancer magazines, one modified for five rounds and the other holds up to eight round. The stock is a Magpul rifle stock and the buffer assemble is a standard rifle set up.

This is my fourth build. The first two were a .300 Blackout and a .223 Rem and worked with no problems. The third was also from JSE and in 6.5 Grendel with a heavy barrel. The only problem with it was finding the right magazine which turned out to be an E-Lander 17 rnd magazine. The 6.5 Grendel is quite accurate but not as accurate as my CZ 527’s.

The first response suggested trying other powders which I did. Five rounds each were loaded using Hornady 225 grain FTX bullets and the maximum amount of powder recommended by Lyman. Powders used were Lil’Gyn, IMR-4227, H-110 and Accurate #9. All but Lil’Gun had the same problems with cycling. Lil’Gun correctly fired all five, however the powder burned very very hot. One would not want to hold the barrel after five rounds and it is wise to wait a minute before picking up the cases.

It also suggested opening up the gas port a bit but I do not like making changes that cannot be undone. However checking the port size and for obstructions is OK. There were no obstructions and the size is .089/.090. When removing the gas port block I noted that the set screws were loose and it appeared the gas block had slid forward about 1/8”. The gas block was moved back and the set screws re-tightened. I then loaded five rounds with the 225 gr. FTX bullet and 41.0 grains of IMR-4227 (1 grain below maximum) which fired with no problem.

The next day sixteen rounds were loaded with the same load and fired. The first magazine full fired OK. The second round of the second magazine did not cycle properly. Checking the set screws found them lose. The gas block was moved back into position and the set screws re-tightened and the remaining round fired OK.

I guess the next step is to replace the gas port block with one that is secured in a better fashion. I am looking at one that is clamped into position rather than with set screws.

Once that is done I will post the results.

Thanks.
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Re: Incomplete cycling

Postby plant_one » Sat Sep 08, 2018 12:36 pm

glad you found your problem.

is your barrel dimpled for a set screw style block? i use those on almost all my builds, and i have yet to have my set screws come loose and allow my block to slide around.

i would suggest taking your calipers and measuring both the OD of the GB journal, as well as the ID of your block. i also always look for a GB that fits the journal snugly - if it'll just drop over the journal, its WAY WAY WAY to loose. ive purchased name brand blocks that were several thousandths out of spec (too loose) before. i'm just shocked that so few shots are causing your set screws to come loose.

how much torque are you putting on those set screws? if your block is aluminum it should be right around 20 in/lbs. if you dont have a FAT wrench already i highly recommend one. it takes the guess work out.

you may also wish to try some loc-tite on your set screws. a drop of blue, or possibly even purple may be just what you need to prevent those from backing out when you dont want it. i use the purple on mine because its what i have on hand (came with my scope lapping/mounting kit i have) and i have yet to have an issue with them coming loose.
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Re: Incomplete cycling

Postby Hoot » Sat Sep 08, 2018 9:28 pm

I've always used gas blocks made from some kind of steel. Either blackened/nitrided SS or CrMo. That way they expand as a function of temperature, more like the steel barrel does.
I have worked on 2 AR's over the years, which had their gas blocks skim bedded. Real beyotch to get loose, but not a hint of leakage or looseness. Have considered doing it on a couple of my own that have gas marks around them from slight leakage. Of course, some blue LocTite will keep those screws where they belong also.

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Re: Incomplete cycling

Postby Quill » Sun Sep 09, 2018 2:52 pm

Thanks for the responses.

The barrel does have a dimple for the rear set screw (I think). It should be pointed out that one of the reason for thinking a clamp on gas block is in order is that the first time the set screws were re-tightened I noted that tightening the rear set screw first just caused the block to be pushed forward about the 1/8 inch. Tightening the front screw first then the rear allowed the gas block to remain in position. Thinking back on it I guess the front set screw is the only one holding the block in position.

I have checked out a couple clamp on gas blocks some of them being adjustable. I am wondering if there are any benefits to be derived from an adjustable block. Also the reason the block was just loosened not completely removed right away is the muzzle break would also have to be removed. The original muzzle break was replaced with one from Windham Weaponry which while not reducing the recoil to any great extent it did greatly reduce the muzzle jump. It is installed with a crush washer which is suppose to be replaced each time the break is removed. When I determine which gas block then the muzzle break will be removed and replaced using my one spare crush washer.

As for tightening the screws to so many inch pounds. The only torque wrench I have runs from 10 to 150 ft/lbs. It works great on my tractor.

Thanks for the inputs.
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Re: Incomplete cycling

Postby Jim in Houston » Sun Sep 09, 2018 3:31 pm

If you're firearm cycles correctly on factory ammunition, then the only variable is the reloaded ammo, and the only departure from "standard" reloading practices for the 450 Bushmaster is the choice of powders. Try some reloads with 38 gr of Lil Gun.
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Re: Incomplete cycling

Postby Quill » Sun Sep 16, 2018 2:39 pm

Received the new gas port block and gas tube for the rifle in the mail yesterday. It was installed in the afternoon without any problems. Five rounds of Hornady factory ammo were fired with no problems. The gas port stayed firmly in position.

Today fifteen rounds were loaded with Hornady 225 gr bullets and IMR-4227 powder. Five each of the rounds had 38.5 gr, 40.0 gr and 41.5 gr of powder. They were fired starting with the heaver loads expecting that if there were any problems with cycling it would occur with the lighter loads. The 40.0 gr and 41.5 gr loads had about two cycling problems each. However the 38.5 gr loads functioned fine with no cycling problems. Also the gas port stayed in position and a check of the clamping screws showed them to be as tight as when they were installed.

This leaves me to wonder if IMR-4227 is one of those powders that does not like to be compressed. When seating the bullets in the two heaver loads the increased pressure required to compress the powder could be felt.

The next step, after ordering more bullets, is to check the performance with H-110 which according to the manuals has about the same performance.
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