Rapier
04-17-2012, 11:14 AM
Some of you guys might be aware that a year ago I got a 6.5 Grendel upper for my AR lower with the idea of using the Grendel as a sub sonic wild hog masher for night shooting.
It did not take to long to discover that there were no sub sonic loads for the Grendel that functioned the AR action while remaining subsonic. It was also discovered that the third component, accuracy, was missing from the other two. So I set about working on the problem and succeeded last Saturday in bringing the three together with one load. To my knowledge it is the first time it has been done, where all shots measured below the super sonic threshold.
Here is the data I posted on the 6.5 Grendel Web site Sunday:
After many attempts to get full function from the Grendel, with 100% function while maintaining 100% sub sonic performance, I achieved the feat on 4-14 by seating the .267 bullet further down in the converted (fireformed) 7.62x39 WW case to an OAL of 1.794 with 12gr of AA-2015 BR under a 160gr Hornady .267 not .264 bullet. I reduced the buffer to 3oz and used a full rifle stock spring in a rifle stock. Temperature on this day 82 degrees. The barrel is 16 inches with a CAR gas tube. The load shoots .5 inches at 50 yards. It shoots 1.5 inches at 100 in a fair wind. At 100 with my 1-9 twist gun, the bullets are starting to get unstable and show signs of striking the target just off center, with round holes, but with ¼ moon black on one edge of the holes. I sighted in my gun for the load and feel confident in making a neck shot on a hog out to 100. Remember this is a night gun, not a day gun, so any day shooting will be with full power loads.
The following is based on a five shot string, the AA-2015 load of 12 gr at 1.794 OAL, delivers an average velocity of 1054 fps, a SD of 31 fps, a high of 1100.7 fps (my SS break is at 1116 fps), a low of 1024 fps with an ES of 77 fps with a MHD of 22.7 There was no failure to function in my 16 inch barrel with the suppressor attached. All shots were with the can. It was very nice to hear the whap as each bullet hit the cardboard at 100.
As we have gone through this process it has become obvious to me that the subsonic loads are going to have a high ES number regardless of what is done, so the figure you should be looking at in your testing is that one shot, that gives you the highest velocity reading. Could you reduce the high fluctuation, yes, and I have done this with bolt guns using spherical buffer. But in a gas gun, in my opinion, based on many years experience with the use of various buffers, using something that does not burn is a recipe for disaster in the future as whatever you use, will build up in the tube/carrier key. An experiment for another day might be to use a very, very slow burning powder as a buffer.
Regards the soot build up, the neck of the 12gr loaded cartridges seals fairly well, leaving the body of the case only moderately dirty.
I am going to add on to this post today 5-4-12 that I was asked on the Grendel forum to test the load for penetration. Well in looking around I found I had a lack of just about anything to shoot. Coming from a small town, the newspaper is very small and I do not drink stuff in gallon jugs. Then I saw the phone books and gathered up 11 phone books. I decided by prior experience that dry paper is more difficult to shoot through than wet so I stacked the books up on a bank and backed off a ways. Whack with the 160gr using the can. I started removing book by book expecting to find the bullet about 6 books in. Well, when I got to 11 and turned it around, it had an exit hole about 1 inch in diameter. I found the bullet 6 inches inside the dirt bank. It penetrated 11 1/2 inches of phone books and 6 inches of dirt. I think a neck shot on a piggy is pretty safe, but the background now becomes important as the 6.5 arrow does not want to stop.
It did not take to long to discover that there were no sub sonic loads for the Grendel that functioned the AR action while remaining subsonic. It was also discovered that the third component, accuracy, was missing from the other two. So I set about working on the problem and succeeded last Saturday in bringing the three together with one load. To my knowledge it is the first time it has been done, where all shots measured below the super sonic threshold.
Here is the data I posted on the 6.5 Grendel Web site Sunday:
After many attempts to get full function from the Grendel, with 100% function while maintaining 100% sub sonic performance, I achieved the feat on 4-14 by seating the .267 bullet further down in the converted (fireformed) 7.62x39 WW case to an OAL of 1.794 with 12gr of AA-2015 BR under a 160gr Hornady .267 not .264 bullet. I reduced the buffer to 3oz and used a full rifle stock spring in a rifle stock. Temperature on this day 82 degrees. The barrel is 16 inches with a CAR gas tube. The load shoots .5 inches at 50 yards. It shoots 1.5 inches at 100 in a fair wind. At 100 with my 1-9 twist gun, the bullets are starting to get unstable and show signs of striking the target just off center, with round holes, but with ¼ moon black on one edge of the holes. I sighted in my gun for the load and feel confident in making a neck shot on a hog out to 100. Remember this is a night gun, not a day gun, so any day shooting will be with full power loads.
The following is based on a five shot string, the AA-2015 load of 12 gr at 1.794 OAL, delivers an average velocity of 1054 fps, a SD of 31 fps, a high of 1100.7 fps (my SS break is at 1116 fps), a low of 1024 fps with an ES of 77 fps with a MHD of 22.7 There was no failure to function in my 16 inch barrel with the suppressor attached. All shots were with the can. It was very nice to hear the whap as each bullet hit the cardboard at 100.
As we have gone through this process it has become obvious to me that the subsonic loads are going to have a high ES number regardless of what is done, so the figure you should be looking at in your testing is that one shot, that gives you the highest velocity reading. Could you reduce the high fluctuation, yes, and I have done this with bolt guns using spherical buffer. But in a gas gun, in my opinion, based on many years experience with the use of various buffers, using something that does not burn is a recipe for disaster in the future as whatever you use, will build up in the tube/carrier key. An experiment for another day might be to use a very, very slow burning powder as a buffer.
Regards the soot build up, the neck of the 12gr loaded cartridges seals fairly well, leaving the body of the case only moderately dirty.
I am going to add on to this post today 5-4-12 that I was asked on the Grendel forum to test the load for penetration. Well in looking around I found I had a lack of just about anything to shoot. Coming from a small town, the newspaper is very small and I do not drink stuff in gallon jugs. Then I saw the phone books and gathered up 11 phone books. I decided by prior experience that dry paper is more difficult to shoot through than wet so I stacked the books up on a bank and backed off a ways. Whack with the 160gr using the can. I started removing book by book expecting to find the bullet about 6 books in. Well, when I got to 11 and turned it around, it had an exit hole about 1 inch in diameter. I found the bullet 6 inches inside the dirt bank. It penetrated 11 1/2 inches of phone books and 6 inches of dirt. I think a neck shot on a piggy is pretty safe, but the background now becomes important as the 6.5 arrow does not want to stop.