175 SMK Length Variance and Seating Depth Question

[Infinity]

Hi All,
I am finally starting to load more than 10 rounds at a time now that I have found my desired load, but during my first 50 round box, I've come across a little speed bump.

I have been loading to 2.810" average COL (variance was 2.806" to 2.8155"). After my first 24 rounds in the box, I had to dip into a different batch of bullets. For the remaining 26 rounds in the box (the new bullet batch), COLs vary from 2.125 to 2.820 (same ogive/seating die setting, and yes it is tight). Since I am loading for FAL mag fit in my LAR-8, I obviously need all of these rounds to fit in the mag. All of my hand loads with this recipe (and I assume these loads are no different, haven't tested yet) shoot an average of 2570 fps out of my 20" barrel. I would really hate to have to tweak my load at this point if I have to increase seating depth, although I would if I had to.

Question time: Is there a linear relationship between seating depth changes and velocity? I am quite a bit under my tested max load (1.8 grains) so I'm not going to make anything explode by increasing seating depth .005", although I would do it incrementally to be safe. Is it possible to make those 26 rounds mag length without making velocity differ too much from the rest of the box? Would the best choice be to trim meplats of the already loaded cartridges? Is that possible? Please feel free to give any recommendation/insight you may have. Thank you much,
Adam

 

[Aimsmall55]

Load to the ogive for the smks. You'll be fine.

 

[Dogtown]

Yep, start measuring COAL to ogive, it's a far more consistent method as the tips (meplat) are rarely uniform anyway. Sinclair makes a line of gauge inserts that hook into a pair of calipers for measuring to ogive as well as shoulder.

Sinclair Insert Style Bullet Comparator | Sinclair Intl

 

[Infinity]

I will get that comparator to confirm the measurements although I'm certain that isn't going to determine if my rounds fit and function in the magazine. It does seem like something I need though. I'll drop an update in here once I'm able to confirm is those longer COL rounds function in my mags. Thanks,
Adam

 

[ChrisGarrett]

Question time: Is there a linear relationship between seating depth changes and velocity? I am quite a bit under my tested max load (1.8 grains) so I'm not going to make anything explode by increasing seating depth .005", although I would do it incrementally to be safe. Is it possible to make those 26 rounds mag length without making velocity differ too much from the rest of the box? Would the best choice be to trim meplats of the already loaded cartridges? Is that possible? Please feel free to give any recommendation/insight you may have. Thank you much,
Adam

I would guess that seating depth variations don't affect velocities in a linear fashion. Seating a bullet too deeply, can spike pressure, but a .005" difference, when you're within normal parameters already, not so much so.

There are going to be so many other factors that will affect velocity, in your reloading process, that I wouldn't worry about things too much.

Depending on your bullets, you might have a seating depth variation of +/- .003" from the ogive, so don't sweat that part.

You can trim your meplats and you can point the tips. Trimming them will lower the BC by 2%, I think it is?

You need to make things mag length safe, so that's where you have to be.

Chris

 

[Infinity]

Chris, thank you. I find your posts insightful and very helpful.