I wanted to create this thread to help new and old shooters with some tips and tricks I have learned through testing and experience. In my opinion and experience, seat length is the biggest factor in a load and rifle's group size. Powder charge controls your velocity and seating depth controls your accuracy, for the most part. There are of course rules to this and there are exceptions, but generally speaking, seating depth controls your group size and pattern if everything else is in check.
Here I have posted 3 pictures in consecutive order, from worse to best. All 3 of these groups were fired from the same rifle, same shooter, same load, other than seat length change. This was out of a 100% stock Remington 700 SPS Stainless 7mm Rem Mag with the only modifications being a free floated barrel with aluminum pillars in the stock and Devcon bedding. The first target shows a target face with a 2" large circle with a 1" small circle in the center. The other two targets are different and have a 1" outside circle and 0.500" inside circle. This is just a reference. Pay attention to how the shots begin to tighten up though.
What we see here is the difference in 3 different seating depths. You can see how the group slowly gets tighter as the seat length changes. In this case the further we worked back the tighter it got. There's a total change of 0.010" in seat length between these 3 targets. The first is 0.040 off, the second is 0.045 off, and the third is 0.050 off. The final group could possibly see improvement with changing seating depth 0.002 shorter or longer, but for a factory rifle we were satisfied with this. This rifle shot all over the place with anything closer than 0.040" off the rifling.
I recommend doing a "rough" seating depth test starting 0.010 off the rifling and working back in 0.010 increments until you find a group that is noticeably tighter. I typically recommend working from 0.010 off to 0.050 off, if you don't have magazine restrictions and other issues. When you find the length that shoots best, shoot groups with bullets that are 0.005 longer and 0.005 shorter. If you still aren't satisfied or just want to tinker, work in and out in 0.002 increments.
I highly, highly recommend using a Hornady OAL gauge and bullet comparator to measure your ammo from your cartridge base to bullet ogive. This will give a much more accurate length than measuring from cartridge base to bullet meplat (tip). I can further post on this with other rifles, but my point is, don't give up on a rifle, bullet, powder, or load until you tune your seat length.
Here I have posted 3 pictures in consecutive order, from worse to best. All 3 of these groups were fired from the same rifle, same shooter, same load, other than seat length change. This was out of a 100% stock Remington 700 SPS Stainless 7mm Rem Mag with the only modifications being a free floated barrel with aluminum pillars in the stock and Devcon bedding. The first target shows a target face with a 2" large circle with a 1" small circle in the center. The other two targets are different and have a 1" outside circle and 0.500" inside circle. This is just a reference. Pay attention to how the shots begin to tighten up though.
What we see here is the difference in 3 different seating depths. You can see how the group slowly gets tighter as the seat length changes. In this case the further we worked back the tighter it got. There's a total change of 0.010" in seat length between these 3 targets. The first is 0.040 off, the second is 0.045 off, and the third is 0.050 off. The final group could possibly see improvement with changing seating depth 0.002 shorter or longer, but for a factory rifle we were satisfied with this. This rifle shot all over the place with anything closer than 0.040" off the rifling.
I recommend doing a "rough" seating depth test starting 0.010 off the rifling and working back in 0.010 increments until you find a group that is noticeably tighter. I typically recommend working from 0.010 off to 0.050 off, if you don't have magazine restrictions and other issues. When you find the length that shoots best, shoot groups with bullets that are 0.005 longer and 0.005 shorter. If you still aren't satisfied or just want to tinker, work in and out in 0.002 increments.
I highly, highly recommend using a Hornady OAL gauge and bullet comparator to measure your ammo from your cartridge base to bullet ogive. This will give a much more accurate length than measuring from cartridge base to bullet meplat (tip). I can further post on this with other rifles, but my point is, don't give up on a rifle, bullet, powder, or load until you tune your seat length.
