Rifle sensitive to C.OA.L.

[fat but funky]

HI Guys,

I have a new rifle that I am developing loads for. Winchester 1885 in 6.5 Creedmoor

Initial loads were bullet testing - Sierra Game King, Hornady Interlock, Hornady SST, Speer HotCor, all 140 grain

While the SGK was best (3 overlapping holes at 100 yards), the interlock was only slightly larger, and the SST and Hotcor were both around 1 inch.

went with the SGK. Today was testing seating depths. Made triplicate cartridges at 0.005" increments.

I found really large variations in group size. Shooting from longest to shortest cartridges groups went from 2" to 0.5" to 1.3" to 0.4" to 1.5" then back down to about 0.5" (to be fair I pulled a shot in the last group which I didn't count here). The big groups were of the 3-equally-spaced-holes variety. Not a tight pair and a flier. So, combined with the uniformly good groups with a range of bullets at a single seating depth makes me think these groups are 'real' and not just a fluke.

I'm not the most experienced reloader, but I have followed this routine a few times for more than a few rifles. I have never had swings this extreme for such small variations in seating depth.

Anyone seen this sort of thing before?

Was using 39 grains of H4350 in twice-fired nosler brass if that makes any difference.

Thanks

Fat

 

[mustang979]

Did you measure to the ogive or to the tip?

 

[todbartell]

Changing seating depth is effecting pressure, thus velocity, and therefore barrel time, which is why you're seeing variations in accuracy. It's harmonics. I find adjusting powder charges to tune accuracy more worthwhile

 

[Ganderite]

The 6.5 bullet has a very long bearing surface, so I would expect to see more sensitivity than, say, a 308 155 gr bullet.

That said, I have not ever seen the kind of result you are quoting.

I suggest you re-do the test in just 3 increments. 20 thou off; 5 thou off and firm contact with rifling. Shoot 5 of each - slowly, with a cool down between groups. The rifle should show a preference, although the SGK is quite forgiving.

 

[Lead hammer]

My 6.5 Creedmoor was easy, set to mag length and done. My 270 was picky with Hornady STT’s. Only shot about 6 tests to find the right one though. The group Range was from 5/8”- 2 1/2” group size.

 

[diopter]

3 shots may not be enough.
Read this and test in similar fashion.

http://www.bergerbullets.com/getting-the-best-precision-and-accuracy-from-vld-bullets-in-your-rifle/