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Old May 22, 2021, 01:04 PM   #1
Join Date: February 24, 2020
Location: Wisconsin
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.243 AR-10 Slow burning Powder issues? General issues with .243 AR10?

A friend is building a AR-10 .243 for me. Aero upper and lower, 22" custom Mcgowan barrel. I hand load for .243 bolt action with RL19 and IMR4831. Guy at the local reloading supply store advised I may have issues with having enough pressure to cycle the rounds properly due to the slower burning powders that I am currently using. Can anyone shed more light on issues that I may have to keep an eye on with my .243 build? Is this a common occurrence? Can I just load hotter rounds (closer to max, or max) or would I need to use a faster burning powder? Thanks in advance for your assistance.

Last edited by Natty270; May 22, 2021 at 01:35 PM.
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Old May 22, 2021, 06:10 PM   #2
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the pressure for the system should be dictated by the location and size of the gas port. As long as your rounds are within spec, unless the gas port size is wrong, I would think it should function fine.

Cool build BTW.
I don't believe in "range fodder" that is why I reload.
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Old May 23, 2021, 02:08 AM   #3
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I've read about it on forums-- but I've never had an AR barrel that was "under gassed"--usually they are at least slightly overgassed (there's a reason why there's been a proliferation in adjustable/tuneable gas blocks made). McGowen has made some AR10 barrels for me the same length as the one you are getting--I usually specify rifle+2 gas length; what are you getting? Unless you're firing a reduced load (which has it's own safety issues) I wouldn't sweat it (at least until you've actually fired it)--especially if the GS guy doesn't have the same set-up and experience with it. One area that builders often overlook IMO when putting AR 10's together is the quality of the bolt and firing pin--it pays to spend extra bucks on these to get something like JP's high performance bolt and firing pin.
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Last edited by stagpanther; May 23, 2021 at 02:21 AM.
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Old May 23, 2021, 07:31 AM   #4
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+1 on the JP bolt. Every AR-10 I've built had cycling issues till I at least put in a JP Improved Extractor , so just buy the whole BCG from them to start with.
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Old May 23, 2021, 08:44 AM   #5
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My bolt guns likes IMR 4064 and 760 for 75 and 87gr bullets. I would be very hesitant to use slower powders in an AR. I wouldn't go slower than IMR 4350 or RL-17 with 95-100gr Hunting bullets.
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Old May 23, 2021, 10:51 AM   #6
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Enhanced bolts are the ticket for .308 pattern ARs. Mine is a DD and I have had JPs in the past.

As stag inferred, most ARs (AR15 and .308 pattern both) are overgassed. With the "specs" being all over the place for the .308 patterns, there is a higher likeliehood of not having a well tuned system out of the gate. Heavier buffers, springs, adjustable gas blocks etc are common ways to mitigate the issue.

With the .243 Win in a .308 pattern AR, bullet velocity is higher, which, regardless of gas length, is going to decrease dwell time over a similarly set up .308. But the bore will also have a lower volume of gas than the .308, so in most cases, it is a wash.

With .308 pattern ARs, I tend to use faster powders, not only for function, but for barrel life. The gas port, regardless of location, size and if it is angled or not, is usually the first location of erosion which, at some point robs velocity and accuracy. I have tested twin barrels with AA2200 and Varget, and the Varget always erodes out and loses accuracy before the AA2200.

I have 5 .243s in bolt and .308 pattern and sadly, I found that it just does not work to use similar loads in both. Generally, the fastest powder you have with load data will work best. The barrel and bolt will last the longest and the function will be more reliable.
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Old June 10, 2021, 03:03 PM   #7
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Thanks for all of your responses fellas. I have some H4350, so i'll work up some loads to break in the rifle barrel with that powder. I'll let you boys know how it goes. I am still waiting on the barrel from McGowen. I ordered it a week shy of 4 months ago, so i'm hoping it will be done soon so I can get some lead downrange. Thanks again!
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Old June 10, 2021, 08:40 PM   #8
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Slower powders get their best performance from longer barrels. Essentially, they push "slower but longer".

Gas operated actions are a balancing act of several factors at the same time, and are generally set up for the "common" load levels, in terms of both overall pressure AND port pressure. Unlike most piston systems, the direct impingement system of the AR design has somewhat unique requirements, and this is why adjustable gas blocks have become common, useful, and sometimes needed modifications to the original design.

I fully expect that if you run the slowest burning powders in an AR pattern .243 you will likely need to "tune" the gas system for best performance. An adjustable gas block seems to me to be the simplest and cheapest potential item to do that with, though you may also need to experiment with different springs and or buffers, as well.
All else being equal (and it almost never is) bigger bullets tend to work better.
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Old June 28, 2021, 03:25 PM   #9
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AR-10/.243 Update

I got my barrel last Wednesday and my buddy came down and finished putting together my new AR10/243 yesterday.

I cleaned the barrel, lapped the scope rings, and bore sighted my new 6x18x40 Leupold VX Freedom CDS 30mm w/Leupold cantilever mount. Commenced to break in the barrel. I shot a total of 5 rounds using Hornady 95 grain SST/New Federal Cases/Fed210m primers/RL-19 43gr. Running around 2940 with my Xbolt same barrel length.

The weapon cycled without issue! But the rounds are ejecting at about 2 o'clock which my buddy said was due to being overpassed. Talked to him about an adjustable gas block like you boys were mentioning. Brass looks fine. I'm going to keep shooting it and monitor. Any thoughts other than an adjustable gas block?

Just took another 2 shots after cleaning the rifle again, and the second shot was a bull at 100. I can tell this is going to be a fun gun to shoot!

Not much copper, if any, in the barrel when cleaning. So far i'm very impressed with the Mcgowen barrel and how the SST's like the barrel.

Thanks again fellas.
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