Not sure what you mean by 'low pressure' with SAAMI spec for Grendel at 52K lbs. .223 is 55K which is not much more. You can run calibres like 223 hotter and the military do this with 5.56NATO at 62K. The new Grey Tip ammo is even hotter than the M855 and all this gives better results down range but the cost is that guns wear out and break down quicker. The reason Grendel is slightly lower is related to the larger surface area of the case pushing back on the bolt, which is hardened but still will crack eventually.
I used to run 223 in a Rem700 bolt action at about 90K lbs, which is way more than SAAMI. We all did, and is the only way you can get a 80gn .223 bullet to 1,000yds with any hope of being accurate. Barrels are changed every year after about 1,000rds and cases last about half they would otherwise. The action is big, strong and chunky. The barrels thick at the chamber. The bolts were after-market with smaller .062" firing pins and tighter firing pin holes to prevent donuts breaking off the primer, which is also a thick magnum CCI450. All this makes it safe. You try that in an AR and it will blow up on the first or second round, maybe in your face.
The 120Nosler BT: although they claim is a varmint bullet is slow to expand, even at higher velocities. They call it "Controlled expansion". It does not matter how fast you send it into flesh from a Grendel it will still slowly expand, and that may be what you want - greater penetration, less disintegration. If you want more expansion then maybe a different bullet is the go, or at least the lighter and therefore faster 100NBT.
Yes, you need a chronograph when loading. The more information you can glean when load testing will make it safer and more efficient.
Nosler recommends 27.5gn of 8208 as max for 120NBT's (@ 1.243OAL). You can free up some case space by making the OAL slightly longer and still fit in a SS mag but you are risking much. At 27.5 the case is full. Add any more and you get into a compressed load. Compounding this is that 8208 is fast compared to other popular powders used in Grendel. This means a steep weight/pressure curve suddenly gets even steeper once you compress it. Manufacturer tables are conservative so you can squeeze a little more in but if you don't have a good handle on internal ballistics you won't be respecting your gun, and if you miss the signs you will be dribbling all over your bib in the hospital.
The Grendel is a relatively slow calibre, just accept it. The case is not big enough to push these big rounds at the optimum speeds they were designed to expand. Add to that the length of the barrel (16") is not conducive to high velocity and you are stuck pushing a heavy bullet at slow speed. If you want speed and not range then you need a different calibre.
So, if you load hot with 8208 you will break the gun. If you want speed go a lighter bullet and/or a longer barrel. If expansion results are still not good enough go a larger case 6.5 of which there are plenty of options: Creedmore, 260Rem, 6.5*47, 6.5*55, 6.5PRS or 6.5*284. If you want that in an AR it will have to be a bigger action - an AR10. Unfortunately there is no slack left at the raggedy edge of a calibre designed to be at the edge of what an AR15 can handle, and no margin for error when dealing with high pressure.
Let me sum this up in one word...
KABOOOOM!
(I use a slower powder with the 120NBT: H4895, using 8208 for lighter bullets).
I used to run 223 in a Rem700 bolt action at about 90K lbs, which is way more than SAAMI. We all did, and is the only way you can get a 80gn .223 bullet to 1,000yds with any hope of being accurate. Barrels are changed every year after about 1,000rds and cases last about half they would otherwise. The action is big, strong and chunky. The barrels thick at the chamber. The bolts were after-market with smaller .062" firing pins and tighter firing pin holes to prevent donuts breaking off the primer, which is also a thick magnum CCI450. All this makes it safe. You try that in an AR and it will blow up on the first or second round, maybe in your face.
The 120Nosler BT: although they claim is a varmint bullet is slow to expand, even at higher velocities. They call it "Controlled expansion". It does not matter how fast you send it into flesh from a Grendel it will still slowly expand, and that may be what you want - greater penetration, less disintegration. If you want more expansion then maybe a different bullet is the go, or at least the lighter and therefore faster 100NBT.
Yes, you need a chronograph when loading. The more information you can glean when load testing will make it safer and more efficient.
Nosler recommends 27.5gn of 8208 as max for 120NBT's (@ 1.243OAL). You can free up some case space by making the OAL slightly longer and still fit in a SS mag but you are risking much. At 27.5 the case is full. Add any more and you get into a compressed load. Compounding this is that 8208 is fast compared to other popular powders used in Grendel. This means a steep weight/pressure curve suddenly gets even steeper once you compress it. Manufacturer tables are conservative so you can squeeze a little more in but if you don't have a good handle on internal ballistics you won't be respecting your gun, and if you miss the signs you will be dribbling all over your bib in the hospital.
The Grendel is a relatively slow calibre, just accept it. The case is not big enough to push these big rounds at the optimum speeds they were designed to expand. Add to that the length of the barrel (16") is not conducive to high velocity and you are stuck pushing a heavy bullet at slow speed. If you want speed and not range then you need a different calibre.
So, if you load hot with 8208 you will break the gun. If you want speed go a lighter bullet and/or a longer barrel. If expansion results are still not good enough go a larger case 6.5 of which there are plenty of options: Creedmore, 260Rem, 6.5*47, 6.5*55, 6.5PRS or 6.5*284. If you want that in an AR it will have to be a bigger action - an AR10. Unfortunately there is no slack left at the raggedy edge of a calibre designed to be at the edge of what an AR15 can handle, and no margin for error when dealing with high pressure.
Let me sum this up in one word...
KABOOOOM!
(I use a slower powder with the 120NBT: H4895, using 8208 for lighter bullets).