The only load data for AR Comp Ive found is Speer. Speer shows a maximum charge of 25.3 grains of AR Comp with the 120 Grain Gold Dot. Speer shows a maximum charge of 24.4 grains compressed charge with a 140 grain Hot Core SP. The 120 grain Gold Dot is a bonded core bullet that build pressures faster than a typical cup and core bullet.
It appears the chamber is on the short side seeing how your having to seat the bullet deep. In new unfired brass in a brand new barrel I would probably use a lower starting charge. Speer shows a minimum charge of 22.8 grains AR Comp with the 120 grain Gold Dot. I think Id try 24.0 grains and go up in .3 grain increments. Your in uncharted territory with a new build and Id use a chronograph while working up.
I think AR Comp is a very desirable powder for the 6.5 Grendel but there is very limited published verified pressure test data for AR Comp. Speer shows a C.O.A.L. of 2.140 with the 140 grain hot core bullet. I think the SP profile is similar to the Hornady SP so the main difference is a longer bullet body of the Speer. With the small case capacity of the 6.5 Grendel longer bullets impede how much powder you can get into the 6.5 Grendel. New brass supposedly has less case capacity compared to fired brass and is another reason Id start with a lower charge. I suspect 25.0 grains of AR Comp will be close to a max load with a 129 grain InterLock SP.
It appears the chamber is on the short side seeing how your having to seat the bullet deep. In new unfired brass in a brand new barrel I would probably use a lower starting charge. Speer shows a minimum charge of 22.8 grains AR Comp with the 120 grain Gold Dot. I think Id try 24.0 grains and go up in .3 grain increments. Your in uncharted territory with a new build and Id use a chronograph while working up.
I think AR Comp is a very desirable powder for the 6.5 Grendel but there is very limited published verified pressure test data for AR Comp. Speer shows a C.O.A.L. of 2.140 with the 140 grain hot core bullet. I think the SP profile is similar to the Hornady SP so the main difference is a longer bullet body of the Speer. With the small case capacity of the 6.5 Grendel longer bullets impede how much powder you can get into the 6.5 Grendel. New brass supposedly has less case capacity compared to fired brass and is another reason Id start with a lower charge. I suspect 25.0 grains of AR Comp will be close to a max load with a 129 grain InterLock SP.