Will changing my seating depth throw off my numbers... a lot?

[Ned Ludd]

Laurie - I recently tested some Juggernauts in a .308 load at various distances off the lands, touching, and into the lands. As compared to jumped bullets, all loads seated at touching or into the lands exhibited a consistent increase in velocity of 15-17 fps (also measured with a LabRadar). Not a huge increase in velocity/pressure, but detectable nonetheless. Perhaps the much larger ogive radius and less abrupt ogive to bearing surface transition of the 183 SMK might to some degree mitigate the pressure effects?

Lefty - I think you will find over the long run that getting a little more velocity out of your load will be an improvement, but you will likely need to re-tune it from scratch. I would suggest using Varget or H4895 as a powder. IMR4895 is a little on the fast side for 175s, even faster than H4895, and somewhat less temperature-stable. Because it sounds as though you aren't limited to loading to mag length for this endeavor, I'd start the charge weight testing at about .015" or .020" off the lands. Once you have found the desired charge weight window, you can go back and optimize the seating depth window as you originally described in this thread. Based on the measurements you provided, starting them at .015" to .020" off the lands should be close to about halfway between where they used to be seated and touching the lands. You should be able to cover that region in a seating depth test with only minimal effects on velocity due to changing the effective (usable) case volume.

 

[Laurie]

Laurie - I recently tested some Juggernauts in a .308 load at various distances off the lands, touching, and into the lands. As compared to jumped bullets, all loads seated at touching or into the lands exhibited a consistent increase in velocity of 15-17 fps (also measured with a LabRadar). Not a huge increase in velocity/pressure, but detectable nonetheless. Perhaps the much larger ogive radius and less abrupt ogive to bearing surface transition of the 183 SMK might to some degree mitigate the pressure effects?

That's my take on it too Ned. It looks like the longer the bullet nose and gentler its curvature as seen in terms of calibres radius, the less pressure change it generates in terms of being in or out of the lands.

Even your 15-17 fps Juggernaut increase is fairly modest in pressure terms whilst if one applies the QuicklOAD 'shot-start' pressure guidelines, there is a much larger computed Pmax increase. (To get 15-17 fps out of a 'jumped Juggernaut' with VarGet in a 30-inch barrel, QL says FWIW that it's the equivalent of a 0.3gn powder increase producing another 1,240 psi PMax. Not to be ignored if working on the pressure limit, but not that great. Conversely, increasing the 'shot start pressure' by the amount QL recommends for a bullet seated 'in' sees a more than 9,000 psi PMax increase in this hypothetical load. Maybe so for a true jammed bullet of course!)

I've got to say I was really surprised by the nil MV change results I got though, also if to a lesser extent with the consistent POI on the paper.

I've never used Berger's recommendations on VLD COALs - ie start 'in', then change to jumping them by increasing amounts in large steps. I have a fair few 168gn 7mm Berger VLDs that won't shoot for toffee seated 'in' in my 7-08 long-freebore F/O rifle, and I mean to try Berger's recommendations exactly next year to see if I can rectify this. It'll be interesting to see how / if MVs change here too. (I've also invested in a couple of hundred 162gn Hornady ELD-Ms for this rifle despite the 162 Amax never having done much in it. Bryan Litz confirms the ELD's significant ballistic improvement over the AMax to an impressive 0.877 G7 form factor / 0.327 average BC from barely noticeable shape changes, but it is a true VLD with a 17.29 calibre radius secant nose form and 0.50 Rt/R value.)

 

[Ned Ludd]

That's my take on it too Ned. It looks like the longer the bullet nose and gentler its curvature as seen in terms of calibres radius, the less pressure change it generates in terms of being in or out of the lands.

Even your 15-17 fps Juggernaut increase is fairly modest in pressure terms whilst if one applies the QuicklOAD 'shot-start' pressure guidelines, there is a much larger computed Pmax increase. (To get 15-17 fps out of a 'jumped Juggernaut' with VarGet in a 30-inch barrel, QL says FWIW that it's the equivalent of a 0.3gn powder increase producing another 1,240 psi PMax. Not to be ignored if working on the pressure limit, but not that great. Conversely, increasing the 'shot start pressure' by the amount QL recommends for a bullet seated 'in' sees a more than 9,000 psi PMax increase in this hypothetical load. Maybe so for a true jammed bullet of course!)

I've got to say I was really surprised by the nil MV change results I got though, also if to a lesser extent with the consistent POI on the paper.

I've never used Berger's recommendations on VLD COALs - ie start 'in', then change to jumping them by increasing amounts in large steps. I have a fair few 168gn 7mm Berger VLDs that won't shoot for toffee seated 'in' in my 7-08 long-freebore F/O rifle, and I mean to try Berger's recommendations exactly next year to see if I can rectify this. It'll be interesting to see how / if MVs change here too. (I've also invested in a couple of hundred 162gn Hornady ELD-Ms for this rifle despite the 162 Amax never having done much in it. Bryan Litz confirms the ELD's significant ballistic improvement over the AMax to an impressive 0.877 G7 form factor / 0.327 average BC from barely noticeable shape changes, but it is a true VLD with a 17.29 calibre radius secant nose form and 0.50 Rt/R value.)

I also was surprised by the very modest increase in pressure/velocity. I did adjust the Start Pressure setting in QuickLoad to account for the velocity increase and found the increase necessary was also modest. The Jugs that exhibited this increase were seated at touching, .003", .006", and .009" into the lands. I suspect a "hard jam" of .020" to .025" into the lands might have produced the more significant pressure spike and change in velocity I had been expecting based on existing anecdotal information. However, I had no reason to seat them that far into the lands and so did not test it.