Velocity & OAL

[Dangermouse]

Like many I have read lots re the dangers of changing the OAL of a round and the consequent dangerous rise in pressures.

As I was unhappy with the grouping of the 185gn Berger's I decided that I would act upon the general consensus that they appear to like a jump of 20 or 30 thou.

My starting/previous OAL was 2.150" Deciding to take the process steady, I loaded 47.3 gn of Viht 550 & CCI BR primers at .005 stages starting at 2.130" and fired 10 each over the chrono from the bench. My barrel is 32" long and is of course .308 cal.
Ovenpaa can note that despite his best efforts, the chrono is still working and that rain water on the sensors must have been the reason for the previous outings error messages, not his attempt at murdering his chrono.

2.130 = ave 2775fps
2.125 = ave 2779fps
2.120 = ave 2783fps
2.115 = ave 2799fps?
2.110 = ave 2785fps

2785fps is a good velocity for the distances I shoot, so I will stay with that one for the time being.


So not the great leaps in velocity that I was expecting between loads. All cases were checked for pressure and all appeared OK. This does surprise me and I may start to ease up a bit on making sure each case is exactly the OAL that I am aiming for.

Having said that I am still struggling to get small average spreads. The lowest was 22fps for the 2.110 and that was marginally the smallest group.
The highest was 55fps and this is despite taking a painfully long time over each load, dropping the last few kernels in by trickler.
I am starting to wonder about my case preparation and am wondering if I am going to have to invest time and money in making sure all the neck variables are taken care of. I seriously do not want to have to spend more time on my brass and would have liked to have written these procedures off to mind games.

At 114 yards all the groups were in the 1" range. Not at all impressive, but before I continue to kick my self I will remember that those Short Siberia tables/chairs are useless as far as I am concerned in achieving accuracy. With a different position/grip with each shot. I really need to get prone to get the best out of the kit.
I was also aiming at patches placed around the target, as these are not the smallest aiming mark, they also will have contributed to larger groups.
Excuses excuses etc,

DM

[Gun Pimp]

DM,

Loading into the lands will raise pressures - slightly - as you have found. But your OAL? 2.110 - 2.130 - surely we are looking at around 2.8 inches for a loaded round?

But, I never load into the lands for F Class - if the Bisley RO calls an emergency 'cease fire' he may want you to open your bolt. If you pull the round - powder in the chamber - shoot ruined.

BTW, 25fps is quite satisfactory for a velocity-spread over five rounds. Lower spreads are nice but 20 -25 fps is OK.

No need to get obsessive with your brass prep. if you are using Lapua. Don't mess with the necks but keep an eye on overall length. Factory chambers will allow your brass to grow by an alarming amount but not necessarily so with a custom reamer/chamber. For example my 308 chamber is 2.02 inches - only about 5 thou. longer than a new case, so I trim all my brass back to 2.00 inches right off. Mik will tell you your chamber length off his reamer print.

Vince

[Dangermouse]

I am measuring to ogive, I,m surprised that no one has come up with different names for the two measurements,

DM