7mm RM - Same charge, longer COAL, lost 100fps

[skahunter831]

Hi all, I'm trying to figure out what happened here. I think I know, but I would like the insight of more experienced shooters.

I'm working up a load for Nosler Accubonds, 160gr, for my Tikka 7mm (D18, 22" barrel). I settled on H1000 (due to many recommendations and its performance (and availability)). A couple weeks ago, I shot five three-shot groups with charges ranging from 68.8 to 70.0 grains in 0.3 grain increments (pressure was fine). Depth was set to SAAMI COAL of 3.290" (which I discovered later is 0.145" off the lands, see below). It was a little breezy, but I still wasn't thrilled with the grouping. The 69.1 grain load shot best with a group size of 0.86 inches. Groups got consistently larger as the charge increased, with the widest group at 1.27 inches with 70 grains of powder. I was not taking enough time between shots, and the barrel got too hot, but I didn't see vertical stringing, the groups were just opening up. The 69.1 grain charge got an average velocity of 2,968 FPS. I'm plenty happy with that speed, but wanted to tighten up the groups.

So I decided to do a bullet seating depth test. I measured the max COAL (touching lands) using GunBlue490's method using a cleaning rod, and got an max COAL of 3.425". I kept the charge at 69.1 grains of H1000, and tested five depths from 0.100 to 0.25 off the lands. Three shots each. The groups were all quite tight, with the one exception of the .100 from lands being 0.998". The rest ranged from .439" to .643", with the best being 0.025" off which grouped 0.439", and a close second at .070" off lands which grouped at .448". Basically identical for my shooting. Ambient temps were about the same (maybe 5 degrees difference)

But here's the thing... the average speed of those rounds, all with 69.1 grains of H1000, was almost 100 FPS slower than the one with 69.1 grains at SAAMI COAL (0.145" off the lands). The average speed for all 18 rounds was 2,870 FPS. The max speed was with the .070" off lands loads, which was 2,905 FPS. From .100" off to 0.25" off lands, the average speeds were, respectively, 2,849, 2,828, 2.905, 2,888, 2.873, and 2.874 FPS.

After a bit of research, I'm GUESSING that the reason was that a shallower seating depth results in less case volume, resulting in lower pressures and velocity. It seems that reducing case volume (deeper seating) can increase pressure, but so can seating the bullet too close to the lands, reducing jump. So did I hit a sweet spot between too close and too far that resulted in lower pressures and lower velocity? Or is something else going on? Or is this all just noise and I need to do more testing?

Where would I go from here? I'm thinking do some charge testing at .070" off the lands, and maybe .025" off as well. Any other recommendations?

A pic of the results from today is attached.

Thanks!

 

[SDHNTR]

You are chasing your tail and 3 shot groups aren’t telling you anything. Too much random variance wrt group size and fps average.

Assuming you are using the same lot of powder…. (If not, there’s your problem)

Shoot a 10+ shot group at .025” off and shoot a 10+ shot group at .100”. I bet your fps averages will be close.

Yes, longer coal can reduce pressure (up until you’re jammed) but a very small amount and not 100 fps worth.

 

[Chris in TN]

All else being equal, seating longer increases volume, reduces peak pressure and ergo reduces velocity. I try - at least generally - to figure out the max length I can shoot then back 0.060"ish off that and leave it there. Whether any given rifle might shoot oh so slightly better at some slightly shorter or longer length I do not know and do not care. The differences are meaningless and you'd burn a barrel half out trying to find the 'sweet spot' if such a thing exists.

There's evidence, but not proof, that you can start seating about 0.060" off the lands, and usually get good results.

Bullet Jump: Is Less Always Better?

About 2 years ago, Mark Gordon shared some preliminary research he’d conducted with the Applied Bal...

precisionrifleblog.com

ETA: also, plugging your specifics into Gordon's Reloading tool using its default case capacity, etc, 70.0 H1000 is over max pressure but indicates a velocity of 3002.6' at 3.325" and a velocity of 2971.8' at 3.4".

Obviously, that's a model, based on a bunch of assumptions, but I have generally found it to be fairly useful. So....it's saying you should be seeing about a 30' difference in muzzle velocity between the longer and shorter seating depth.

 

[skahunter831]

Assuming you are using the same lot of powder…. (If not, there’s your problem)

Shoot a 10+ shot group at .025” off and shoot a 10+ shot group at .100”. I bet your fps averages will be close.

Yes, same lot. Thanks, good reminder that this isn't enough data to really know anything. Might try that.

...you can start seating about 0.060" off the lands, and usually get good results.

ETA: also, plugging your specifics into Gordon's Reloading tool using its default case capacity, etc, 70.0 H1000 is over max pressure but indicates a velocity of 3002.6' at 3.325" and a velocity of 2971.8' at 3.4".

Obviously, that's a model, based on a bunch of assumptions, but I have generally found it to be fairly useful. So....it's saying you should be seeing about a 30' difference in muzzle velocity between the longer and shorter seating depth.

Thanks, I think I'll try a few tests at .060" or .070" off with a few different charges centered around the 69.1 grain mark. What was the reported pressure with 70.0 grains, if I might ask? I attached a few pics of one of the 70gr cases, I don't see any pressure issues, do you? This was the second loading of this case.

 

[Chris in TN]

^^That's what it shows. Again, there's a ton of assumptions and at least generally if GRT tells me that a load generates max pressure and, for example, 3075'MV, if I try that load and see no signs of pressure and my actual velocity is, say, 3025'......I personally don't mind increasing the powder charge until I hit that original max speed of 3075'. Or whatever it was. My assumption there is that GRT might miss my actual pressure and velocity because I'm not carefully inputting throat lengths and other particulars, but it likely *will* very closely predict the actual max speed I can get, and I generally feel comfy enough working up to that speed.

As always, all models are incorrect. Some are useful. I think GRT is useful.

 

[skahunter831]

Thanks, very helpful. I've seen GRT before but never used it until last night. It's very interesting and fun to mess around with. One thing that caught my eye was case volume. Playing with that input dramatically affects pressure (as is logical, but a lot more than I expected). Because I was using 70 grains and not seeing any pressure, I wondered if my case volume was over SAAMI spec, and indeed it appears to be! I did a volume measurement using a syringe to fill ten fired cases, five each of Nosler and Norma. The Norma cases were all within 0.2 grains of each other, and all 5+ grains over spec, 87 vs 82! Nosler were all at least 3 grains over spec (85 grains), again with a variation of about 0.2 grains.

Even keeping SAAMI COAL at 3.290", with 70 grains of H1000, the 85 grain volume of the Nosler brass kept the pressure under SAAMI max. Going to 87 grains of volume (the Norma brass) and my current planned COAL of 3.365" (0.070" off lands), the pressure dropped to 89% of max pressure. If I drop the charge weight to 69.1 gr (current "favorite" load), the pressure drops to 85% of max pressure, with velocity around 2884, or about 20 FPS less than my small sample size of 3 rounds (which averaged 2,905).

That's probably enough testing for my purposes (elk hunting). I also value barrel life, so don't see a need to push pressures if I'm already getting 2900+ FPS. I'll prob shoot another 10 or 20 rounds with 69.1 grains at 0.070" off the lands to confirm my grouping, and might also shoot another 10 or 20 with 70 grains just to see if I can eke out another 50-75FPS, but that difference is only about 0.5" of drop at 300 yards and 1" at 400 yards.