Seating depth - does it even matter?

[Harvey_NW]

The first group is 10 shots with a cbto of 1.480". The second is 9 shots at cbto of 1.483" and the third is 9 shots at cbto of 1.486". All shots were fired round robin style.

Pretty interesting results, I would say.

Interesting to say the least. This is the first time I've seen that much variance with decent sample sizes, especially with such little change. Thanks for shooting the shots and posting your results.

 

[pbroski]

It's the greatest variance I've seen as well. I've done a decent amount of testing recently and I believe seating depth makes a measurable difference in at least some gun/ammo combinations.

The main test rifle is a .223 Howa with the factory Stocky stock topped off with a Nightforce NXS 2.5-10x42 firing 73 gr. ELD-M with H4895.

I've done 10 shot strings at 100 yards and 825 yards and I'm generally seeing about a 25 percent improvement between a good cbto vs. a bad one. If I get time I might post some more results.

 

[Ice-kub]

How many more

I did a seating depth test today with my CZ527 American in .17 Hornet. The scope is an old Leupold Vari X III 6.5-20x40 EFR.

The load is Hornady brass, Hornady 15.5 gr NTX bullet, CCI 400 primer and VV N120 powder. Velocity is approximately 3975 fps. The range was 99.7 yards fired off of a front and rear sandbag rest.

The first group is 10 shots with a cbto of 1.480". The second is 9 shots at cbto of 1.483" and the third is 9 shots at cbto of 1.486". All shots were fired round robin style.

Pretty interesting results, I would say.

BTW, it was my first time using my new Garmin chronograph. It picked up every single shot where as the LabRadar I used previously had not picked up even a single shot. I'm very impressed with the Garmin.

I'm curious if it has something to do with the case volume. 17 hornet is a pretty small cartridge compared to say a 300 Wm. That 0.006 difference as a % of case volume of the hornet vs a Wm would be quite the difference no? I haven't done the math but seems like it could be enough?

 

[wind gypsy]

Any load that is that sensitive about 3 or 6 thou seating depth can eff right off. That is wild.

 

[pbroski]

Yes it's small but is still a bottleneck cartridge. The bullet diameter/case capacity ratio is on par with many standard size cartridges. According to QuickLoad a change of .006" of seating depth will only change the velocity by about 5 fps, so not much difference.

 

[pbroski]

Any load that is that sensitive about 3 or 6 thou seating depth can eff right off. That is wild.

That's fine. But if there was that much gain in precision to be had, I think a lot of other people would take it. I know I would.

 

[ddowning]

I would check .003 the other way from the top target. If it doesn't shoot decent, I would test the good load again. If it is really that finicky, I would be interested to see the reamer print.

 

[Harvey_NW]

That's fine. But if there was that much gain in precision to be had, I think a lot of other people would take it. I know I would.

I did leave the post open ended, but I'm still taking your results with a grain of salt. I would assume that such a small caliber bullet with very short bearing surface could be subject to something like in bore tilt with farther jumps to the lands, especially if there was extra clearance in the throat. Like mentioned above, it would be interesting to see the reamer print and know the dimensions.

This combination might actually be a case where seating depth does matter, but I would question the consistency of it lasting for long, or being a "chase the lands" and always tweaking situation.

 

[pbroski]

It's a factory barrel, so I don't have a reamer print. The .17 Hornet is a relatively new creation by Hornady, so the tolerances are likely pretty tight. What I can tell you is that at a cbto of 1.486", the bullet is only about .008" from the lands, so not much room for tilt.

I would load at 1.482". It might be a few hundred shots before it wears itself out of the node. I'll monitor throat erosion to actually see what happens.

I'm also going to test some more loads, going shorter in length.