6mm Creedmoor gas gun

[RH300UM]

I'm in the process of acquiring parts for a long range hunting/practice gas gun build.
Hardest part is the barrel. Wanted a 22-24 inch 7-7.5 twist as I want to run pills in the 105 ish weight range. All pre chambered tubes are usually short throated so I was looking elsewhere so I could get a custom chamber reamed. Most barrel suppliers are 8 months out
Any suggestions?

If I go pre cut chamber I would probably go with a Proof all steel barrel

 

[Deleted member 115360]

Just an idea, but look at the 6mm arc.

Way cheaper to build and shoot. Barrels are readily available, ar15 instead of ar10. Cheap available factory ammo, and they are wonderful. Maybe 150fps slower than a 6cm with 5 times the barrel life or more. A 24" barrel with shoot a 108gr eld-m about 2750 with factory ammo. It will push a 105gr about 2,800. Mags feed great, less recoil, way less weight.

 

[budlight]

I just went looking for 6 CM ar barrels and none to be found. It just wasn't a commercial success. I've been a 243 varminter for many years. Plentiful cases and a wonderful round. As to twist 8 is fine unless you are going for 115 gr

 

[RH300UM]

Just an idea, but look at the 6mm arc.

Way cheaper to build and shoot. Barrels are readily available, ar15 instead of ar10. Cheap available factory ammo, and they are wonderful. Maybe 150fps slower than a 6cm with 5 times the barrel life or more. A 24" barrel with shoot a 108gr eld-m about 2750 with factory ammo. It will push a 105gr about 2,800. Mags feed great, less recoil, way less weight.

Got one already

 

[ARlife4me]

Got one already

I have a 243win in AR10 and the throat was very short even for sierra 90gr fmj, like .028" short contacting lands. Bought a uni-throater from ptg for the remedy. I noticed with winchster 100gr sp (factory ammo) it would barely chamber. I bought uni-throats for most calibers I own (24, 26, 30 and 33cal).

 

[budlight]

I have a 243win in AR10 and the throat was very short even for sierra 90gr fmj, like .028" short contacting lands. Bought a uni-throater from ptg for the remedy. I noticed with winchster 100gr sp (factory ammo) it would barely chamber. I bought uni-throats for most calibers I own (24, 26, 30 and 33cal).

That is a new one to me. I've assembled custom 243, 260, and 308 ar10's All you have to do is follow the saami max length when reloading, Unlike a bolt gun! 2.710 max for 243

https://saami.org/wp-content/uploads/2019/02/ANSI-SAAMI-Z299.4-CFR-Approved-2015-12-14-Posting-Copy.pdf

 

[ARlife4me]

That is a new one to me. I've assembled custom 243, 260, and 308 ar10's All you have to do is follow the saami max length, Unlike a bolt gun! 2.710 max for 243

https://saami.org/wp-content/uploads/2019/02/ANSI-SAAMI-Z299.4-CFR-Approved-2015-12-14-Posting-Copy.pdf

Sierra data @ 2.650" for 90gr fmj, but short throat would only get 2.622" against the lands. From 75 - 105gr they are all short. A gunsmith said they (barrel manufacturer) might have intentionally left it like that? Maybe for 1 who wants a specific length throat?

 

[Shawn Carverelli]

I'm in the process of acquiring parts for a long range hunting/practice gas gun build.
Hardest part is the barrel. Wanted a 22-24 inch 7-7.5 twist as I want to run pills in the 105 ish weight range. All pre chambered tubes are usually short throated so I was looking elsewhere so I could get a custom chamber reamed. Most barrel suppliers are 8 months out
Any suggestions?

If I go pre cut chamber I would probably go with a Proof all steel barrel

my PR carbon fiber barrel. it'll shoot 103 eldx and 108 mk . precut chamber. oal is right @ 2.790.So far best group was 87 vmax using reloader 17 and cci450 primers.The CF barrel and the titanium bolt carrier lightened it up consierably. Then I went and pit on a 31 oz vortex 5-25x pst gen2 scope

 

[cgbills]

With any high pressure round like a 6 or 6.5 CM, an adjustable gas block is a must. Look at the superlative arms block, as it vents excess gas out the front. This reduces chamber pressure and cuts town on brass damage

 

[budlight]

With any high pressure round like a 6 or 6.5 CM, an adjustable gas block is a must. Look at the superlative arms block, as it vents excess gas out the front. This reduces chamber pressure and cuts town on brass damage

IMO - That is a very uninformed view of how thing really are. The informed person buys or has made rifle length or even + 1 or 2 inch gas tubes and the size of the gas port hole is important. So I had AR's built with small gas port holes at the
+2 inch using the slower powders at over 62,000 psi with the light weight buffer weights and springs. 5.56, 243, 308

I have 3 rifles with superlative arms adjustable gas block pistons and find them to have lots of drawbacks like they go out of adjustment with extended use or nearly come a part. I've improved the original design by adding a Adams arms spring to the BCG piston area

 

[cgbills]

Yes do not forget the +2 gas, especially with a 22+ inch barrel. The longer barrels delay the release of the gas out the barrel and increases the amount of pressure that goes down the gas system. +2 gas taps later and allows for the pressure to fall of quicker. The first 6.5CM AR10 I built years ago was a 22in JP with rifle gas and it would destroy brass. I have since built 22in 6.5CM rifles on +2 gas and the results have been much much better

 

[budlight]

Yes do not forget the +2 gas, especially with a 22+ inch barrel. The longer barrels delay the release of the gas out the barrel and increases the amount of pressure that goes down the gas system. +2 gas taps later and allows for the pressure to fall of quicker. The first 6.5CM AR10 I built years ago was a 22in JP with rifle gas and it would destroy brass. I have since built 22in 6.5CM rifles on +2 gas and the results have been much much better

That is not how rifles work. right after the primer is set of it ignites the powder. You have several items that determine max pressure achieved. Max pressure is achieved in the first few inches of bullet travel down the barrel. Slower powder is slightly farther down the barrel.

But anyway if you drilled a holes every couple inches starting at 6 inches clear down the barrel and put pressure sensors. You are going to see a decrease in pressure with distance even before the bullet leaves the barrel.


The reason is as the bullet travels the volume of the tube increases lowering pressure along with the gases cooling.

Well anyway you can read up on 3 gas tube lenghts VS pressure. In a 5.56 the carbine length hole gets hit with 30,000 psi mid length was 18,000, and rifle length was clear down to 13,000. Or something along those lines of pressure. So + two inches the pressure is even lower. All my builds are rifle length at a minimum with small gas port holes. You also get higher FPS because you are not bleeding off so much pressure

I previously posted a chart from a barrel manufacturer about the sizes of gas port holes. A smaller hole way down the barrel eliminated the need for even adjustable gas blocks. I have multi AR's with non adjustable gas blocks and just tune them with the spring and buffer weight.

 

[ARlife4me]

That is not how rifles work. right after the primer is set of it ignites the powder. You have several items that determine max pressure achieved. Max pressure is achieved in the first few inches of bullet travel down the barrel. Slower powder is slightly farther down the barrel.

But anyway if you drilled a holes every couple inches starting at 6 inches clear down the barrel and put pressure sensors. You are going to see a decrease in pressure with distance even before the bullet leaves the barrel.


The reason is as the bullet travels the volume of the tube increases lowering pressure along with the gases cooling.

Well anyway you can read up on 3 gas tube lenghts VS pressure. In a 5.56 the carbine length hole gets hit with 30,000 psi mid length was 18,000, and rifle length was clear down to 13,000. Or something along those lines of pressure. So + two inches the pressure is even lower. All my builds are rifle length at a minimum with small gas port holes. You also get higher FPS because you are not bleeding off so much pressure

I previously posted a chart from a barrel manufacturer about the sizes of gas port holes. A smaller hole way down the barrel eliminated the need for even adjustable gas blocks. I have multi AR's with non adjustable gas blocks and just tune them with the spring and buffer weight.

The AP barrels I have don't destroy brass in 6.5creed with a rifle length system.
port sizes per barrel lengths from AP site
20"/.0760"
22"/.0730"
24"/.0700"
All 3 barrels have non-adjustable blocks, standard weight buffers and springs. None pop primers or have any other issues. I wish I could say that for the +2 gas barrels using standard weight recoil parts even with and adjustable block. 5.56's are a no brainer along with 308win's on gas systems, but other cartridges don't play the same game on pressures per length. Take 3 creedmoors of different calibers as the case is the same length. Start by using the same powder (probably won't be able to use the same amount) and pressures will not be the same at each per inch. Yes, powder volume and type changes the way it'll act also as you know. Regardless of 65k at or within 2-3 inches of throat will have a drastic difference between even 260rem and 6.5creed as max pressure between the 2 are different. I speak this as my experience with these cartridges and with the same barrel company (Proof) with 260rem and 6.5creed. As you stated about gas in the barrel is no doubt true as is why bolt actions give higher velocities than AR's with the same barrel lengths for comparison. Another reason most factory ammo doesn't function well between the 2 in AR's as it's built for bolt guns.

 

[Hydra6]

They are out there. Seen multiple 6CM "gas guns" at PRS matches. Good luck!