CXR,
I agree with 52, BENCHMARK is fast. Arguably too fast for the Grendel and therefore inefficient relative to other powders when loading safely. It is not a popular powder because you can get higher muzzle velocities using slower powders. This is no doubt why your factory loads are faster than Lee's tables for BENCHMARK, not because they are hotter but because they use more efficient powders.
Be also aware as Greyfox was saying that powder tables come from 24" test barrels with strain gauges. You can add another 100fps to the same load if you swapped your 20" barrel for 24" - roughly 25fps/inch.
BENCHMARK is a single-based powder made by Australian Defence Industries (ADI). In Australia it is called BM2, to delineate it from BM1, which is even faster. BM was developed to be a fine grain single-based powder that meters similar to ball powders for the civilian Bench-rest market; hence the name. Hodgdon imports BM2 and re-badges it for the US market calling it BENCHMARK. Hodgdon and ADI's load tables give a clumsy range of powders for the Grendel. It is unclear whether ADI or Hodgdon did the testing but as they both display exactly the same data they are both equally misguided in recommending fast powders for the Grendel. By tabling particular powders it implies they are the most efficient and we seem to have eclipsed them over the years with our community testing.
For single-based/temperature stable powders like H4198 and BENCHMARK are, according to this forum too fast and therefore too inefficient. I would only recommend them for SBR's. Your 20" barrel is better suited for slower powders like 8208 and H4895. For the avoidance of doubt a 'slow' powder refers to burn rate, not muzzle velocity. In the Grendel, faster powders can reach their max safe load with volume to spare that is unusable.
What is 'Safe' depends on the gun you are shooting - how strong it is, and of course your attitude to safety. SAAMI is a good benchmark however you can exceed it if you know what you are doing. Be careful of hot loads in an AR. AR's are not as strong as bolt guns. Your bolt will crack and this is the canary in the coal mine that your loads have exceeded SAAMI and are unsafe in that gun.
If you prefer single-based powders I advise using 8208 for bullet weights up to 120gn and 4895 for 120 and above, in a 20" barrel. You also need a chronograph so you know what's happening in your gun and not someone else's.
If you want a good source of load data across a wide range of powders the manuals 52' is recommending is a neat, comprehensive reference. Plus the Search function of this forum. A ballistics program like Quickload is also very helpful. Most helpful is a combination of all of these and of course, experience. Manufacturers may have the fancy strain gauges but we have the 'broken-bolt gauges' and a community of shared experience. This includes years of constant engagement/interest with the calibre whereas manufacturers have to dilute theirs across many other calibres.
Here's a picture of Hogdon's powders in a range from fast on the left to slower on the right. Note how the kernels get bigger and longer the slower the powder. Your BENCHMARK will meter well but I wouldn't use it.
[ATTACH=CONFIG]16152[/ATTACH]
I agree with 52, BENCHMARK is fast. Arguably too fast for the Grendel and therefore inefficient relative to other powders when loading safely. It is not a popular powder because you can get higher muzzle velocities using slower powders. This is no doubt why your factory loads are faster than Lee's tables for BENCHMARK, not because they are hotter but because they use more efficient powders.
Be also aware as Greyfox was saying that powder tables come from 24" test barrels with strain gauges. You can add another 100fps to the same load if you swapped your 20" barrel for 24" - roughly 25fps/inch.
BENCHMARK is a single-based powder made by Australian Defence Industries (ADI). In Australia it is called BM2, to delineate it from BM1, which is even faster. BM was developed to be a fine grain single-based powder that meters similar to ball powders for the civilian Bench-rest market; hence the name. Hodgdon imports BM2 and re-badges it for the US market calling it BENCHMARK. Hodgdon and ADI's load tables give a clumsy range of powders for the Grendel. It is unclear whether ADI or Hodgdon did the testing but as they both display exactly the same data they are both equally misguided in recommending fast powders for the Grendel. By tabling particular powders it implies they are the most efficient and we seem to have eclipsed them over the years with our community testing.
For single-based/temperature stable powders like H4198 and BENCHMARK are, according to this forum too fast and therefore too inefficient. I would only recommend them for SBR's. Your 20" barrel is better suited for slower powders like 8208 and H4895. For the avoidance of doubt a 'slow' powder refers to burn rate, not muzzle velocity. In the Grendel, faster powders can reach their max safe load with volume to spare that is unusable.
What is 'Safe' depends on the gun you are shooting - how strong it is, and of course your attitude to safety. SAAMI is a good benchmark however you can exceed it if you know what you are doing. Be careful of hot loads in an AR. AR's are not as strong as bolt guns. Your bolt will crack and this is the canary in the coal mine that your loads have exceeded SAAMI and are unsafe in that gun.
If you prefer single-based powders I advise using 8208 for bullet weights up to 120gn and 4895 for 120 and above, in a 20" barrel. You also need a chronograph so you know what's happening in your gun and not someone else's.
If you want a good source of load data across a wide range of powders the manuals 52' is recommending is a neat, comprehensive reference. Plus the Search function of this forum. A ballistics program like Quickload is also very helpful. Most helpful is a combination of all of these and of course, experience. Manufacturers may have the fancy strain gauges but we have the 'broken-bolt gauges' and a community of shared experience. This includes years of constant engagement/interest with the calibre whereas manufacturers have to dilute theirs across many other calibres.
Here's a picture of Hogdon's powders in a range from fast on the left to slower on the right. Note how the kernels get bigger and longer the slower the powder. Your BENCHMARK will meter well but I wouldn't use it.
[ATTACH=CONFIG]16152[/ATTACH]