Just picked up a BCL stripped reciever set and have been doing research on what caliber to go with for my build. 6.5 Creedmoor is superior to 308 WIN but I've read about issues with the 6.5 CM in semiautomatic with feeding issues due to the shoulder angles and bullet seating depth. However while I can find why the problems exist I can't find how the solution has been rectified. I presume the problems have been sorted out since it seems like every manufacturer offers a 6.5 CM variant of their AR-10's. So what are the items to look out for if I decide to make my build with 6.5 CM vs 308.
Issues with 6.5 CM in a AR platform
- Thread starter welder179
- Start date
There are a number of issues that need to be addressed for 6.5CM in a gas gun. This is all based off of what I read from various sources on the net
1. Becuase of the taper/ profile of 6.5 billets, some of the VLD/high BC hollow point bullets do not feed well in gas guns. Having M4 feed ramps will help this but you will still be limited somewhat by COL (cartridge overall length). Shooting tipped/pointed bullets will aid in reliability.
2. Pressure.
A. Firing pin hole/primer flow. Becuase of the higher overall pressure (not just chamber) through the firing cycle, it is common for gas guns using a standard 308 bolt to have issues with the primer cup flowing back into the firing pin hole, which can lead to reliability problems and limitations to reloading with regards to powder charge. The solution for this is to get a bolt that is rated as "high pressure" which typically involves a smaller firing pin hole (to be more supportive of the primer cup) and a smaller firing pin.
B. Gas port pressure. Becuase 6.5 cartridges typically use slower burning powders than 308, there is a higher residual pressure at the gas port which results in a higher pressure in cycling the action which can lead to early unlocking, higher BCG velocity, feeding problems, etc. there are a number of considerations to take into account to remedy this.
I. Gas system length. From what I have read, nothing shorter than a rifle length gas system should be used in any of the NR barrel lengths that you will likely consider to build. Some manufactures will even use an "XL" gas system (rifle length + 2") for barrel lengths longer than 20 inches. The key factor here is "dwell time" which is the time from when the bullet passes the gas port to before it leaves the muzzle. During this time, gas pressure continues to flow back through the gas tube and accelerate the carrier. Basically you want to minimize the dwell time to reduce gas port pressure. This may be done by lengthening the gas system or perhaps some manufacturers will drill smaller gas ports to allow less gas to flow back through the system.
II. Port pressure. The afore mentioned issues can result in a high port pressure which, again will result in a higher BCG velocity that can cause reliability issues. An adjustable gas block will help limit the gas flow back into the system through cycling and reduce BCG velocity. The only downside to using only an adjustable gas block is that on say a 20" barrel with a mid length gas system (not ideal in my opinion), the dwell time will result in a lot of port pressure that will cause premature gas port errosion. This may not be that big a deal in a 6.5 gun because barrel life may too short for this to become an issue.
III. BCG velocity. Barring the first two remedies, high port pressure will result in a very fast moving BCG. This can cause issue with feeding, and premature unlocking. Another way to combat this is to use a special buffer system, such as the JP silent capture spring, heavy buffers, adjustable weight BCGs or full mass BCGs. Tinkering with the weight of the buffer and or BCG can help increase reliability by slowing down the BCG velocity and delaying unlocking. This can be an expensive route to go, but also can yield solid benefits in recoil reduction.
That's just the summation of what I've found. I'm in the process of trying to build a 6.5 CM stag 10 so that's why I've been doing the research. There are some god posts on snipers hide and YouTube for this if you click around.
Good luck!
1. Becuase of the taper/ profile of 6.5 billets, some of the VLD/high BC hollow point bullets do not feed well in gas guns. Having M4 feed ramps will help this but you will still be limited somewhat by COL (cartridge overall length). Shooting tipped/pointed bullets will aid in reliability.
2. Pressure.
A. Firing pin hole/primer flow. Becuase of the higher overall pressure (not just chamber) through the firing cycle, it is common for gas guns using a standard 308 bolt to have issues with the primer cup flowing back into the firing pin hole, which can lead to reliability problems and limitations to reloading with regards to powder charge. The solution for this is to get a bolt that is rated as "high pressure" which typically involves a smaller firing pin hole (to be more supportive of the primer cup) and a smaller firing pin.
B. Gas port pressure. Becuase 6.5 cartridges typically use slower burning powders than 308, there is a higher residual pressure at the gas port which results in a higher pressure in cycling the action which can lead to early unlocking, higher BCG velocity, feeding problems, etc. there are a number of considerations to take into account to remedy this.
I. Gas system length. From what I have read, nothing shorter than a rifle length gas system should be used in any of the NR barrel lengths that you will likely consider to build. Some manufactures will even use an "XL" gas system (rifle length + 2") for barrel lengths longer than 20 inches. The key factor here is "dwell time" which is the time from when the bullet passes the gas port to before it leaves the muzzle. During this time, gas pressure continues to flow back through the gas tube and accelerate the carrier. Basically you want to minimize the dwell time to reduce gas port pressure. This may be done by lengthening the gas system or perhaps some manufacturers will drill smaller gas ports to allow less gas to flow back through the system.
II. Port pressure. The afore mentioned issues can result in a high port pressure which, again will result in a higher BCG velocity that can cause reliability issues. An adjustable gas block will help limit the gas flow back into the system through cycling and reduce BCG velocity. The only downside to using only an adjustable gas block is that on say a 20" barrel with a mid length gas system (not ideal in my opinion), the dwell time will result in a lot of port pressure that will cause premature gas port errosion. This may not be that big a deal in a 6.5 gun because barrel life may too short for this to become an issue.
III. BCG velocity. Barring the first two remedies, high port pressure will result in a very fast moving BCG. This can cause issue with feeding, and premature unlocking. Another way to combat this is to use a special buffer system, such as the JP silent capture spring, heavy buffers, adjustable weight BCGs or full mass BCGs. Tinkering with the weight of the buffer and or BCG can help increase reliability by slowing down the BCG velocity and delaying unlocking. This can be an expensive route to go, but also can yield solid benefits in recoil reduction.
That's just the summation of what I've found. I'm in the process of trying to build a 6.5 CM stag 10 so that's why I've been doing the research. There are some god posts on snipers hide and YouTube for this if you click around.
Good luck!
Wow, that was massively informative, thanks!
If I did go the 6.5 route I would be looking at a 22" or 24" barrel and I have seen the option for +2 gas systems, so that makes sense now.
Would a BCG labeled as 6.5 CM work as you described to handle the higher pressure?
If I did go the 6.5 route I would be looking at a 22" or 24" barrel and I have seen the option for +2 gas systems, so that makes sense now.
Would a BCG labeled as 6.5 CM work as you described to handle the higher pressure?
There are a few. I think the most expensive option is the JP enhanced high pressure bolt. You can buy either just the bolt or the LMOS BCG. Kinda pricey, but from what I've read, worth it.
Option 2 is aero precision. They sell a complete BCG that you can actually FIND in Canada theat is reasonably priced. I think their phosphate and melonite 308 BCGs are either both available in a HP variant or are made stock with the HP firing pin hole/pin.
Their may be other options out there but I've honestly had a hard time finding consistent information on other brands compatability with 6.5 CM.
All the aforementioned bolts are branded as 308, since the case head is the same diameter, which makes it even more difficult to distinguish. Lol.
Personally, I'm going to shell out for the JP bolt and LMOS carrier, but I'm in no rush to complete my build.
Option 2 is aero precision. They sell a complete BCG that you can actually FIND in Canada theat is reasonably priced. I think their phosphate and melonite 308 BCGs are either both available in a HP variant or are made stock with the HP firing pin hole/pin.
Their may be other options out there but I've honestly had a hard time finding consistent information on other brands compatability with 6.5 CM.
All the aforementioned bolts are branded as 308, since the case head is the same diameter, which makes it even more difficult to distinguish. Lol.
Personally, I'm going to shell out for the JP bolt and LMOS carrier, but I'm in no rush to complete my build.
Inside 300m the heavier bullet weights hit harder, sure. But beyond that, 6.5 calibers pretty much dominate. Don't get me wrong, I shoot PRS comps with my 308 and it works just fine. But toe to toe, 6.5 outperforms in terms of external ballistics and terminal ballistics past say 300-400m. Depends on what you're doing I guess. Plus 6.5 is typically less recoil, which is always nice.
