Short Cycling & gas port size *PROBLEM SOLVED*

Talk about the AR15 style rifles chambered in 450 Bushmaster.

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Re: Short Cycling & gas port size

Postby rinkrat311 » Sun Dec 23, 2018 10:04 pm

plant_one wrote: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!


I agree on not opening the port larger than the gas tube ID. Short of an out of spec chamber I'm not sure what else to check at this point. I did a full clean on the rifle today and will do some shooting tomorrow to add to the "break in" round count.

Once the numbered drills arrive I do plan to only go a few thou larger at a time and function check between. I would like to avoid an adjustable gas block if I can.

Stay tuned...
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Re: Short Cycling & gas port size

Postby Hoot » Sun Dec 23, 2018 11:15 pm

rinkrat311 wrote:
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...


I started this reply earlier today but got called away by better half and now I'm a couple of posts behind:

There was a standard gas port spec when there was one make and model of barrel. Now there are more than can be accurately reported and with pistol, carbine, mid-length and rifle length systems, the number of port sizes has grown proportionally.

Keep in mind that the optimal port size usually gets bigger the longer the gas system because the available pressure to be sampled goes down the further down the barrel you take it from.

Here's a pressure visual:

Image

So, if you saw a report of the hole size someone used for a rifle length gas system, it would be necessary to have a larger sample hole than further back where a carbine length gas system would take it from. Ditto for the other length gas systems. Throw in dwell time which if you're unfamiliar with the term, refers how far from the muzzle the sample port is, which reflects how long you have before the bullet breaks the seal as it exits the muzzle (pressure now zero) for that sampled gas to work the action and you can see how this can spiral into a table with one short leg affair. IE there's interactions beyond just pressure to contend with as far as how changing the hole size will impact cycling. The good news is the proper hole size is not a 1 or 0 dimension, otherwise everyone would be pulling their hair out if they loaded up a different weight bullet or a different powder charge weight. The window is wider than that. Properly chosen, the gas port hole size will accommodate a wide range (within reason) of loads in the 450b. That applies the most when the optimal length gas system is employed. If you wind up with a length that's either too short or too long for optimum, the window narrows. For your 16" system, the carbine is close enough to optimum to afford you a lot of forgiveness. For my 20" system, while a carbine will work, the sweeter spot moves toward a mid-length (IMHO) gas system.

Bottom line is what others have said. Changing the port size should be reserved until all else fails or you risk overshooting and having to install an adjustable block. There is beauty in simplicity. When you add another variable, the cause and effect monster raises his head a little higher, adding to the cost of more ammo to evaluate performance. Every adjustable gas block I own is because I went a bridge too far with my port size or wanted to load some odd recipe outside the 9 dots and not beat up my brass. The latter being my 24" 6.5 Grendel. Always starving for a little more power... ;)

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Re: Short Cycling & gas port size

Postby rinkrat311 » Thu Dec 27, 2018 6:01 pm

So I was able to get a hold of a knowledgeable gunsmith at Delta and it was advised to clip coils from the buffer spring to see if that helps the cycling issue. If I have to clip more than 4 coils, I was advised something else is wrong and the barrel mey need returned for evaluation.

Tomorrow I will reassemble the rifle, lube well, and start with one coil clipped off the buffer spring to see what happens... Stay tuned!
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Re: Short Cycling & gas port size

Postby plant_one » Thu Dec 27, 2018 10:44 pm

i hope they plan to send you a new buffer spring. *facepalm*


theres no reason to be cutting springs if you're running the correct *STANDARD* spring in either your adjustable or non-adjustable (a1-a2 style) stock. this is right up there with 'dont drill the gas port unless you absolutely need to' type of gun plumbering.

if you've got the right number of coils (39 and 44 respectably) to match your buffer tube setup leave it alone and find the real problem. especially with a 450 bushmaster. this cartridge has more than enough umph behind it to move a standard buffer spring around. most of us put our energy into trying to slow down the action of the so named thumper, not make cycling easier.....

if you really gotta cross off the buffer spring as the problem - borrow one from a friend.

or just order one for $5. you're gonna need a replacement anyway for the one you just cut 4 coils off to prove you dont have a spring problem. :mrgreen: :mrgreen:
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Re: Short Cycling & gas port size

Postby Hoot » Fri Dec 28, 2018 12:31 am

Image

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Re: Short Cycling & gas port size

Postby rinkrat311 » Fri Dec 28, 2018 10:52 am

I had a feeling the advice I received was a terrible bandaid for the situation... I'm planning to open the gas port slightly next week after the final holiday of the season and will report back with the results. No springs will be harmed in this testing ;)
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Re: Short Cycling & gas port size

Postby Siringo » Tue Jan 01, 2019 2:28 pm

I switched out my Bushmaster barrel for a custom 450 barrel (because I wanted a faster twist). Unfortunately the barrel showed up as a mid-length gas system. I have been monkeying around with this since last fall to get the rifle to function (lock back on an empty magazine and pick up a fresh round). Late fall I was able to get it to function. The gas point was opened to 0.096 inches. BUT I had very little confidence in this rifle like I did with the original barrel. I had used my rifle in conditions below zero degrees Fahrenheit, so this new barrel HAD to function in that temperature extreme. So today in MN it below zero (-7). Last night I put my upper and ammo in the freeze too. I went to my range and first tried to see if the bolt would lock back after firing ----- NOPE!. Tried a loaded magazine ------ it would not pick up a round after firing. Needless to say I am disgusted that I thru good money at a rifle that worked fine before and now I have something I can not rely on. Aftermarket suppliers just don't seem to get it thru their heads that this round produces very low gas pressure at the port. Anything other than a carbine system is asking for trouble. Just my two cents worth.

So next go around is 0.0995 hole size (#39 drill). I would rather be over-gassed than under-gassed.
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Re: Short Cycling & gas port size

Postby rinkrat311 » Fri Jan 04, 2019 7:59 pm

Well I opened the gas port to 0.088" tonight. I ended up using a handheld drill in one of those drill press mechanisms that turns a handheld drill into a drill press. This was not my first choice, I have a large floor standing metal drill press I intended to use. However, after spending 15 minutes getting everything lined up and triple checking my double checking, I realized that my chuck won't hold the tiny drills!!! Needless to to say I was super bummed and more nervous I would have to use the other option but everything went off without incident. I threw a 7/16" dowel down the barrel to act as a stop so I didn't accidentally drill too deep or touch the other inside wall of the barrel.

I re-cleaned the barrel thoroughly and re-assembled the rifle. Tomorrow will be test firing to see if my short cycling issues are solved...
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Re: Short Cycling & gas port size

Postby rinkrat311 » Sat Jan 05, 2019 9:48 am

Opening the port seems to have solved the cycling problems! Bolt holds open on last round and full 5 round mag dumps are possible now. Brass is laying around 3 o'clock consistently and doesn't look beat up from the cycling action. Thank you to all who offered guidance and advice, I'm a happy thumper owner!
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Re: Short Cycling & gas port size

Postby Siringo » Sun Jan 13, 2019 10:22 am

Siringo wrote:I switched out my Bushmaster barrel for a custom 450 barrel (because I wanted a faster twist). Unfortunately the barrel showed up as a mid-length gas system. I have been monkeying around with this since last fall to get the rifle to function (lock back on an empty magazine and pick up a fresh round). Late fall I was able to get it to function. The gas point was opened to 0.096 inches. BUT I had very little confidence in this rifle like I did with the original barrel. I had used my rifle in conditions below zero degrees Fahrenheit, so this new barrel HAD to function in that temperature extreme. So today in MN it below zero (-7). Last night I put my upper and ammo in the freeze too. I went to my range and first tried to see if the bolt would lock back after firing ----- NOPE!. Tried a loaded magazine ------ it would not pick up a round after firing. Needless to say I am disgusted that I thru good money at a rifle that worked fine before and now I have something I can not rely on. Aftermarket suppliers just don't seem to get it thru their heads that this round produces very low gas pressure at the port. Anything other than a carbine system is asking for trouble. Just my two cents worth.

So next go around is 0.0995 hole size (#39 drill). I would rather be over-gassed than under-gassed.


Opened port up as described above. This past week while in Northern MN it was 0 degrees. I left my factory ammo and carbine outside. Carrier was lubed only with light Weapons Extreme oil. Upon firing the action picked up and chambered rounds as well as locked back on the last one.
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