AR10 Adjustable Gas Block?

[kcofohio]

I'm about to order a 20" .308W barrel with a rifle length gas system. If suitable, I'd like to be able to switch from jacketed to cast with gas-checked. The build is in DPMS style.

What do I look for on a barrel as pertaining to the gas port size? I have read where some have enlarged the hole for the adjustable gas block.

This is a bit new to me. What good references do any of you look to. Or am I about to do trial and error? I'd prefer not the error.

 

[Demi-human]

Well, if you do error, you can just put a protective wood or brass rod in the bore and drill the port out with a properly sized # drill bit. By hand or with a press, since it will already be piloted.

My 450Bushmaster wouldn’t cycle with even maximum loads. When I stuck the proper sized bit in the port, I found it wasn’t drilled all the way through. I forgot to place a buffering material in the bore, but managed not to nick the other side when it went through.

Unfortunately, I don’t know offhand what the correct port size would be for a .308.

 

[kcofohio]

---

Well, if you do error, you can just put a protective wood or brass rod in the bore and drill the port out with a properly sized # drill bit. By hand or with a press, since it will already be piloted.

My 450Bushmaster wouldn’t cycle with even maximum loads. When I stuck the proper sized bit in the port, I found it wasn’t drilled all the way through. I forgot to place a buffering material in the bore, but managed not to nick the other side when it went through.

Unfortunately, I don’t know offhand what the correct port size would be for a .308.

Good point about having something to prevent over drilling!

 

[Shawn Dodson]

The problem with .308 barrels is that 7.62x51 ammo is lower pressure than .308 Win ammo, therefore barrel makers usually chose a compromise gas port diameter to function with both.

I have SLR Rifleworks adjustable gas blocks on my .308 uppers. After a shooting session with them I put a drop of oil on the adjustment screw and run the screw in and out a couple of times to prevent carbon lock.

 

[Varminterror]

Ream the port, don’t drill it. Drilling will throw a burr which is more hassle to fix afterwards than is doing the job the correct way from the jump.

Better still - order the barrel from the maker with the right port size and avoid the DIY port reaming all together.

 

[Lennyjoe]

I picked up a PSA .308 upper with an adjustable gas block already installed. Made life a lot easier, but I have no interest shooting cast thru it.

 

[DMW1116]

I’ve turned my gas block (PSA 20” upper) all the way off for subsonic Titegroup loads but I don’t know how well cast would run in an AR10 system, if at all. If it will, then that’s great. I do like the click adjustable block that came with my upper.

 

[Demi-human]

Here is something I stumbled across…

GAS PORT SIZES​

Disclaimer: These are lists of compiled information from multiple sources. While we make our best effort to test and produce these should be considered for reference only. Any time you start messing with the gas port on your rifle you run the risk of destroying the barrel or causing it to be over gassed. It is much better to slowly work your way up to what you would be happy with.

AR-15 Platform 5.56mm
20"	rifle	5.56		0.0935
20"	rifle	5.56		0.096
20"	rifle	5.56		0.098
16"	mid	5.56		0.078
16"	mid	5.56		0.081
16"	car	5.56		0.0625
16"	car	5.56		0.065
16"	car	5.56		0.07
14.5"	car	5.56		0.067
14.5"	car	5.56		0.086
11.5"	car	5.56		0.081
11.5"	car	5.56		0.089
24"	rifle	5.56		0.089

AR-15 Platform 7.62x39mm Conversion
20"	rifle	7.62x39		0.12
20"	rifle	7.62x39		0.125
16"	car	7.62x39		0.078
16"	car	7.62x39		0.081
16"	car	7.62x39		0.0938

DPMS LR-308 Style Rifle
18"	rifle	0.308		0.0960
18"	car	0.308		0.0635
16"	mid	0.308		0.078
16"	mid	0.308		0.081
16"	car	0.308		0.07
16"	car	0.308		0.073
16"	car	0.308		0.076
20"	rifle	0.308		0.0935
20"	rifle	0.308		0.096

AR-15 Platform 300 BLK Conversion Subsonic Ammo
16"	car	300 BLK	UNSUPRESSED	0.12
16"	car	300 BLK	UNSUPRESSED	0.125
16"	car	300 BLK	SUPRESSED	0.106
16"	car	300 BLK	SUPRESSED	0.11
16"	pistol	300 BLK	UNSUPRESSED	0.067
16"	pistol	300 BLK	SUPPRESSED	0.086
9.5"	pistol	300 BLK	UNSUPRESSED	0.093
9.5"	pistol	300 BLK	SUPPRESSED	0.093
8.1"	pistol	300 BLK	UNSUPRESSED	0.104
8.1"	pistol	300 BLK	UNSUPRESSED	0.12
8.1"	pistol	300 BLK	SUPRESSED	0.086
8.1"	pistol	300 BLK	SUPRESSED	0.093
AR-15 Platform 300 BLK Conversion Supersonic Ammo
16"	pistol	300 BLK	UNSUPRESSED	0.0625
16"	pistol	300 BLK	SUPPRESSED	0.059
16"	car	300 BLK	UNSUPRESSED	0.099
16"	car	300 BLK	SUPPRESSED	0.093
9.5"	pistol	300 BLK	UNSUPRESSED	0.073
9.5"	pistol	300 BLK	SUPPRESSED	0.067
8.1"	pistol	300 BLK	UNSUPRESSED	0.076
8.1"	pistol	300 BLK	SUPPRESSED	0.067

The 300 Blackout data has been compiled from Micro MOA

I agree with the use of a reamer, but as a simple carpenter, I hadn’t any, but I do have an extensive set of numbered drills for all the lock sets I install. I just tried a few for good fit. After a quick shop-about online just now, the cheapest .093 reamer I could find was $24. I hadn’t thought of leaving a burr, which is why I always recommend to NOT do it the way I do. I just wanted my cheap rifle to work.
Of course, buying the correctly sized and quality barrel will be the most efficient method.

In addition, I didn’t realize there was such a wide range of port sizes!

Also, I suppose a larger diameter port than necessary will be restricted properly by the gas block adjustments. But would there be a detriment to having a “little pocket” under that gas block? Catch debris from the Cast bullets?

 

[CoalCrackerAl]

If your running cast to save on buying jacketed. These are nice compromise. They can be loaded to 3000 FPS in a 1:10 twist bore. I run them in my 30.06 and 7.62x39 AR'S.

Accura Outdoors Precision Bond Bullets - Accura Outdoors

Accura Outdoors is the home of the Precision-bond copper plated bullet. The most affordable accurate bullet you can buy.

accuraoutdoors.com

 

[Varminterror]

I’d be interested to see how much lead accumulates around the gas port or in the gas system when pursuing this game. Unless a guy is really disciplined about limiting velocities, this can go ugly really quickly, and even at limited velocity (especially if switching back and forth between supersonic jacketed and slower lead), I’m REALLY curious as to what goes on around the gas port. I’d expect leading like the burnout track on a drag strip…

 

[Varminterror]

Any time you start messing with the gas port on your rifle you run the risk of destroying the barrel or causing it to be over gassed.

Destroying the barrel might be a little too alarmist here, and of course, the intent of expanding the port diameter for the purpose of ensuring control-ability by an AGB also paves over the risk of overgassing.

But the table shared there doesn’t reflect appropriate port diameters respective to low velocity cast lead loads. It’s only applicable to standard pressure loads for the respective cartridges, barrel lengths, and gas system lengths.

A riflespeed AGB comes to mind here, for very quick, referenceable, repeatable gas changes. But I’d still be rather wary of swapping back and forth without thoroughly cleaning the bore - and the port erosion reality doesn’t strike me as favorable for swapping in and out of cast.

 

[kcofohio]

I picked up a PSA .308 upper with an adjustable gas block already installed. Made life a lot easier, but I have no interest shooting cast thru it.

I will have to study up on which gas block to use. Hopefully I can gain some insight here on that.

I have a nice mold for 155 gr. bullets. Got it originally for 300 BLK. There is some load data on cast bullets for the .308W using H335 that I hope I can build on.

 

[kcofohio]

Destroying the barrel might be a little too alarmist here, and of course, the intent of expanding the port diameter for the purpose of ensuring control-ability by an AGB also paves over the risk of overgassing.

But the table shared there doesn’t reflect appropriate port diameters respective to low velocity cast lead loads. It’s only applicable to standard pressure loads for the respective cartridges, barrel lengths, and gas system lengths.

A riflespeed AGB comes to mind here, for very quick, referenceable, repeatable gas changes. But I’d still be rather wary of swapping back and forth without thoroughly cleaning the bore - and the port erosion reality doesn’t strike me as favorable for swapping in and out of cast.

Good point of cleaning the bore before swapping types. I powder coat my cast. So if I do my part correctly, I should have minimal to no fouling.

The Riflespeed AGB looks supreme. I like the extended dial. Got it in my favorites for later. Thanks!

 

[kcofohio]

If your running cast to save on buying jacketed. These are nice compromise. They can be loaded to 3000 FPS in a 1:10 twist bore. I run them in my 30.06 and 7.62x39 AR'S.

Accura Outdoors Precision Bond Bullets - Accura Outdoors

Accura Outdoors is the home of the Precision-bond copper plated bullet. The most affordable accurate bullet you can buy.

accuraoutdoors.com

Not so much as the cost of jacketed. But more of the challenge. My plan is to not start off with cast until I get a grip on loading jacketed. I load a lot for .223, but cases are plentiful for that. I want my .308 cases to last longer.

 

[DMW1116]

Just based on the load data for 308 cast in the Lyman Cast manual, I don’t know how many loads will run a 308 semiauto. The powders are different or considerably reduced from standard jacketed loads. Reduced power 30 caliber rifles are kinda the realm of 300 BO and 7.62x39.

 

[CoalCrackerAl]

My 7.62x39 will run on 16 grains of 2400.

 

[DMW1116]

Checking some cast loads, the pressures are often about 60%-65% of the usual jacketed loads, with some as low as the upper end of 38 Special standard pressure. I doubt they will run a gas operating system. Running them with the gas off will shoot them but I don’t know how that would play out in terms of leading and the gas port.

 

[kcofohio]

Here is something I stumbled across…

GAS PORT SIZES​

Disclaimer: These are lists of compiled information from multiple sources. While we make our best effort to test and produce these should be considered for reference only. Any time you start messing with the gas port on your rifle you run the risk of destroying the barrel or causing it to be over gassed. It is much better to slowly work your way up to what you would be happy with.

AR-15 Platform 5.56mm
20"	rifle	5.56		0.0935
20"	rifle	5.56		0.096
20"	rifle	5.56		0.098
16"	mid	5.56		0.078
16"	mid	5.56		0.081
16"	car	5.56		0.0625
16"	car	5.56		0.065
16"	car	5.56		0.07
14.5"	car	5.56		0.067
14.5"	car	5.56		0.086
11.5"	car	5.56		0.081
11.5"	car	5.56		0.089
24"	rifle	5.56		0.089

AR-15 Platform 7.62x39mm Conversion
20"	rifle	7.62x39		0.12
20"	rifle	7.62x39		0.125
16"	car	7.62x39		0.078
16"	car	7.62x39		0.081
16"	car	7.62x39		0.0938

DPMS LR-308 Style Rifle
18"	rifle	0.308		0.0960
18"	car	0.308		0.0635
16"	mid	0.308		0.078
16"	mid	0.308		0.081
16"	car	0.308		0.07
16"	car	0.308		0.073
16"	car	0.308		0.076
20"	rifle	0.308		0.0935
20"	rifle	0.308		0.096

AR-15 Platform 300 BLK Conversion Subsonic Ammo
16"	car	300 BLK	UNSUPRESSED	0.12
16"	car	300 BLK	UNSUPRESSED	0.125
16"	car	300 BLK	SUPRESSED	0.106
16"	car	300 BLK	SUPRESSED	0.11
16"	pistol	300 BLK	UNSUPRESSED	0.067
16"	pistol	300 BLK	SUPPRESSED	0.086
9.5"	pistol	300 BLK	UNSUPRESSED	0.093
9.5"	pistol	300 BLK	SUPPRESSED	0.093
8.1"	pistol	300 BLK	UNSUPRESSED	0.104
8.1"	pistol	300 BLK	UNSUPRESSED	0.12
8.1"	pistol	300 BLK	SUPRESSED	0.086
8.1"	pistol	300 BLK	SUPRESSED	0.093
AR-15 Platform 300 BLK Conversion Supersonic Ammo
16"	pistol	300 BLK	UNSUPRESSED	0.0625
16"	pistol	300 BLK	SUPPRESSED	0.059
16"	car	300 BLK	UNSUPRESSED	0.099
16"	car	300 BLK	SUPPRESSED	0.093
9.5"	pistol	300 BLK	UNSUPRESSED	0.073
9.5"	pistol	300 BLK	SUPPRESSED	0.067
8.1"	pistol	300 BLK	UNSUPRESSED	0.076
8.1"	pistol	300 BLK	SUPPRESSED	0.067

The 300 Blackout data has been compiled from Micro MOA

I agree with the use of a reamer, but as a simple carpenter, I hadn’t any, but I do have an extensive set of numbered drills for all the lock sets I install. I just tried a few for good fit. After a quick shop-about online just now, the cheapest .093 reamer I could find was $24. I hadn’t thought of leaving a burr, which is why I always recommend to NOT do it the way I do. I just wanted my cheap rifle to work.
Of course, buying the correctly sized and quality barrel will be the most efficient method.

In addition, I didn’t realize there was such a wide range of port sizes!

Also, I suppose a larger diameter port than necessary will be restricted properly by the gas block adjustments. But would there be a detriment to having a “little pocket” under that gas block? Catch debris from the Cast bullets?

Thanks for the chart! I missed that yesterday. Good info!

 

[kcofohio]

Checking some cast loads, the pressures are often about 60%-65% of the usual jacketed loads, with some as low as the upper end of 38 Special standard pressure. I doubt they will run a gas operating system. Running them with the gas off will shoot them but I don’t know how that would play out in terms of leading and the gas port.

Reading up over at castboolits.com, I may have to invest in a mold for heavier bullets. The heavies seem to be more reliable for cycling.