Short Cycling & gas port size *PROBLEM SOLVED*

Talk about the AR15 style rifles chambered in 450 Bushmaster.

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Short Cycling & gas port size *PROBLEM SOLVED*

Postby rinkrat311 » Fri Dec 21, 2018 7:29 am

I've been doing a bunch of reading here and elsewhere trying to diagnose my short cycling issues with my recent AR build in 450 bushy. My initial thoughts were that using non-factory magazines was the issue and went through several combinations of magazines and followers without any luck. This week I was able to obtain a factory AR Stoner 5 round magazine for the rifle. I confirmed that an empty magazine would hold the bolt open as it should. When I loaded one round into the magazine, after firing the brass ejected fine but the bolt held open as shown; not clearing the end of the follower in the magazine. I then loaded two round into the magazine, charged one, fired, noticed the bolt closed into battery but upon dropping the magazine confirmed that the second round was still in the magazine.

Bolt open on mag.jpg
Bolt open on mag.jpg (96.49 KiB) Viewed 11847 times


I proceeded to take the hand guard off to look for gas leaks and the gas tube to gas block and gas block to barrel areas look fine with no apparent leaks that could be causing the short stroking. I removed the gas block from the barrel and also confirmed no major leaks and a fairly decent carbon ring around the port. Using some calipers i was able to measure the gas port size and noted it at 0.0775". From my lurking about this site, it seems that this is small and should be opened up to somewhere in the 0.093" - 0.096" range. Can someone confirm this port size and that I'm correct in thinking this is the root cause of my short stroking issues? Thank you for any help provided, this has been a fun project to build and am eager to see this across the finish line!

Gas tube to Gas Block.jpg
Gas tube to Gas Block.jpg (38.67 KiB) Viewed 11847 times

Gas port after Gas Block Removal.jpg
Gas port after Gas Block Removal.jpg (54.11 KiB) Viewed 11847 times

Gas port size.jpg
Gas port size.jpg (67.98 KiB) Viewed 11847 times
Last edited by rinkrat311 on Wed Jan 09, 2019 12:26 pm, edited 1 time in total.
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Re: Short Cycling & gas port size

Postby Al in Mi » Fri Dec 21, 2018 9:48 am

first off, welcome. your barrel carbine or midlength gas?

This last move (1 1/2 whopping miles) has been brutal on finding stuff but ran across some notes on my 16" factory upper lastnite. Carbine length gas, port size .093, has never had a eject or lock back issue with any loads or mags tried to date.

When I broke this upper in, it was litterly dripping wet with oil for 50 or so rounds, then gave it a good cleaning with light oil and she's slick as proverbial snot now. It will fling brass from factory and Lilgun loads 4' at 4:00. My pet 300gr XTP over 45gr 1680 it just sorts dumps them out but bolt still locks back.

so I might suggest trying it wet first, see if that helps, if not, open the port .010 and try that, then 093 if you have to.
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Re: Short Cycling & gas port size

Postby rinkrat311 » Fri Dec 21, 2018 9:55 am

Al in Mi wrote:first off, welcome. your barrel carbine or midlength gas?

This last move (1 1/2 whopping miles) has been brutal on finding stuff but ran across some notes on my 16" factory upper lastnite. Carbine length gas, port size .093, has never had a eject or lock back issue with any loads or mags tried to date.

When I broke this upper in, it was litterly dripping wet with oil for 50 or so rounds, then gave it a good cleaning with light oil and she's slick as proverbial snot now. It will fling brass from factory and Lilgun loads 4' at 4:00. My pet 300gr XTP over 45gr 1680 it just sorts dumps them out but bolt still locks back.

so I might suggest trying it wet first, see if that helps, if not, open the port .010 and try that, then 093 if you have to.


Barrel is 16" carbine length, bull barrel profile.

I've been running the BCG pretty wet through all the different magazines I've been trying when I first assumed it was a mag problem. That's not to say the 40-ish rounds I've shot thus far have even close to broken anything in yet. I may try giving it a deep clean and re-lube and give it another go before putting it on the drill press to open the port.

Thanks for the reply!
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Re: Short Cycling & gas port size

Postby Pathfinder » Fri Dec 21, 2018 10:28 am

X2 on running the new build wetter than normal. Running dry for a few rounds can speed up the break in, but won't help the initial cycling. Wet the BCG down.

A lot of factors can contribute to the short cycle. Don't start drilling until you communicate the issue with the barrel manufacturer.
By the way, who made the barrel ?

I had a 16" barrel that started the same way. I got impatient and opened the port to .08. Rifle then cycled, but started beating the snot out of the case rims and eventually developed extraction issues and short cycling again. Upgraded the extractor and it worked for a little while, then the same issues cropped up.
Turns out it had a badly machined chamber that I hadn't checked.
Replaced it with the same model 16" barrel (0.07 gas port) with a cleaner chamber and it's run through 200 rounds without a cycling issue.
....I really hope I don't regret saying that.....

I could have saved myself a lot of time and aggravation by working with the manufacturer first.
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Re: Short Cycling & gas port size

Postby rinkrat311 » Fri Dec 21, 2018 10:34 am

Pathfinder wrote:X2 on running the new build wetter than normal. Running dry for a few rounds can speed up the break in, but won't help the initial cycling. Wet the BCG down.

A lot of factors can contribute to the short cycle. Don't start drilling until you communicate the issue with the barrel manufacturer.
By the way, who made the barrel ?

I had a 16" barrel that started the same way. I got impatient and opened the port to .08. Rifle then cycled, but started beating the snot out of the case rims and eventually developed extraction issues and short cycling again. Upgraded the extractor and it worked for a little while, then the same issues cropped up.
Turns out it had a badly machined chamber that I hadn't checked.
Replaced it with the same model 16" barrel (0.07 gas port) with a cleaner chamber and it's run through 200 rounds without a cycling issue.
....I really hope I don't regret saying that.....

I could have saved myself a lot of time and aggravation by working with the manufacturer first.


Barrel is made by XX-Treme Barrel Company, purchased through Delta Team Tactical. I have emailed them about the issue with a detailed explanation of the current situation. I also am not going to hold my breath for too long as I know this may be just banging my head against the wall...

I plan to polish the magazine lips tonight if I get time, then clean, re-assemble and run it wet for another mag or so to see if things can improve. Depending on how cranky the kids gets I may just grab a heavy glove and go sit in the basement and watch TV while working the charging handle until my arms hurt
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Re: Short Cycling & gas port size

Postby rinkrat311 » Sat Dec 22, 2018 4:18 pm

I found online that Bear Creek Arsenal sells a barrel very similar to mine in photos and specs except they list a gas port size of 0.145". If I do end up drilling this out, would it be recommended to go a few numbered drills sizes at a time, re-assembling in between to determine if short stroking is solved? Would I got right to 0.094" like I have seen posted here a few times or even larger to 0.145"? Is the a standard gas port size for 16" bushmaster barrels or is it still a bit of a black box for the industry?
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Re: Short Cycling & gas port size

Postby Hoot » Sat Dec 22, 2018 5:58 pm

Its been hard sitting back and watching this thread without wading in.

For a 16" carbine length gas system, if you have to drill the gas port beyond a number 43 bit, then you are missing something that is not right with your rig. Could be:

1) Needs more break-in.
2) Has out of spec chamber. Not likely but I had one once so I threw it out there.
3) Gas tube not aligned with key or not nesting deep enough into it.
4) Wrong recoil spring or buffer.
5) Receiver extension tube not vented enough to prevent air compression
6) Out of spec BHO latch. Was recently reported in another thread.
7) More items but not major contributors to your issue

If you pursue your problem by enlarging your gas port until the problem is resolved, save some time and order an adjustable gas block because one day, when your rig breaks in and smooths out, you'll need it.

Hoot
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Re: Short Cycling & gas port size

Postby rinkrat311 » Sat Dec 22, 2018 9:13 pm

Hoot wrote:Its been hard sitting back and watching this thread without wading in.

For a 16" carbine length gas system, if you have to drill the gas port beyond a number 43 bit, then you are missing something that is not right with your rig. Could be:

1) Needs more break-in.
2) Has out of spec chamber. Not likely but I had one once so I threw it out there.
3) Gas tube not aligned with key or not nesting deep enough into it.
4) Wrong recoil spring or buffer.
5) Receiver extension tube not vented enough to prevent air compression
6) Out of spec BHO latch. Was recently reported in another thread.
7) More items but not major contributors to your issue

If you pursue your problem by enlarging your gas port until the problem is resolved, save some time and order an adjustable gas block because one day, when your rig breaks in and smooths out, you'll need it.

Hoot


Thank you for the reply Hoot. I'll try to break it in more by shooting it, time and money for ammo being the obstacles there. I have a standard milspec buffer and spring in the rig now but I also have a T2 heavy buffer I can swap in if it would help. At this point I half to assume (Yea I know...) the gas tube to gas key interface is ok but I'll find out how to check that. The extension screw is vented and I checked it to allow air flow. I'll also check the BHO latch to see if it is within spec. I have a set of numbered drill bits on order, I will get and adjustable gas block in the mail soon I guess. It's curious that there is no standard gas port size for this cartridge like other more common AR cartridges...
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Re: Short Cycling & gas port size

Postby Pathfinder » Sun Dec 23, 2018 6:03 pm

Hoot's list covers it. Plenty to consider before opening the port.

Just a note, DON'T rely on the gas port size listings you will see on many sites. If you do much comparing you will see two or three different sizes listed for the same barrel. BCA lists that barrel (the one with 0.875" gas block) port size at 0.145", but if you look at the complete upper it shows 0.089", and if you look at the completed rifle it shows 0.070".
I have that barrel and the gas port is actually 0.070". Sees to be running OK.

I googled xx-treme barrel company 450 bushmaster and the 16" barrel that pops up is listed to have a 0.086" port size.
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Re: Short Cycling & gas port size

Postby plant_one » Sun Dec 23, 2018 9:01 pm

as i understand it.... the most you should ever open up a gas port is 0.125 - why you ask? thats the ID of just about all gas tubes. no reason to open up your port anywhere beyond the limiting factor of the system.


however - if you gotta go that far on a 16" carbine... as mentioned you've already got another problem to tend to. almost no ar should need a .125 gas port unless its a VERY VERY VERY purpose built custom barrel. a few manufacturers have done them as production barrels, but it was a cheater way to overcome other shortcomings in the system and often caused more trouble than they "fixed". most of those ended up needing adjustable gas blocks to function well 100% of the time.

if you're going to get the drill bits out, i highly recommend going very slowly until you get functionality. go one step at a time, reassemble, and test fire.

however before you go there, i would 100% make sure that the issue isnt somewhere else in the system.


good luck and keep us in the loop!
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