How much velocity change from brass switch?

[Coloradocop]

I've finally worn out some of my Remington brass for my .308 Win, and started loading some of the many pieces of brass I have from factory Federal GMM ammunition.

Anyway, I expected I might see some small changes in dope from the brass change, but the changes I saw at the range the other day seemed a little bit excessive. I thought I'd post the experience here, just to see if this seems like a normal range of change for a switch in brass.

Normally I dial 4.3 mils for center-of-plate at 600 yards. This time I was overshooting the plate with that adjustment. I dialed down to 4.1 mils, and started scoring hits high on the plate (hit the mounting holes twice, and between them a couple of times). When I moved to the 500 yard line I had similar experiences with my elevation needing to be dialed lower to even hit high on the plate.

I decided to check case volume today, just to see if there was a difference between the two brands of brass. The Remington brass held just 1.0 grains more water than the Federal brass, and weighed 10 grains less than the Federal brass. That lower case capacity in the Federal brass should theoretically yield faster velocities with that brass (as I saw), but I'm a bit surprised if that 1.0 grain of case capacity would result in a 0.3 mil change in dope at 600 yards.

I'm still using the same primers, and the same 44.4 grains of Varget powder (though from a new can). Bullets are the same. Temperature was within 10 degrees of what I shot at on my previous range trip (40-50 degrees F), and I shot at the same range as before.

I didn't have a chronograph with me, but all of the hits suggested that my bullet was moving faster than before.

What do you guys think?

 

[Captain Kirk]

Re: How much velocity change from brass switch?

Less case capacity with the same charge = higher pressure resulting in higher velocities. Lower your charge by .2-.4gr and see where your at, it really helps to know your velocity, if you could load up a few using your Rem brass and run it over a chronograph, it would be easy to duplicate the velocity with the new brass/load.

 

[Coloradocop]

Re: How much velocity change from brass switch?

I've already chronographed the Remington brass load in the past, and figured that based on my chronograph readings (and dope chart) I should have been running about 2,620 fps with that load.

I know that lower case capacity increases pressure, and thereby velocity, but it seemed like it increased by a surprising amount, given that I could only fit one additional grain of water weight into the Remington case than the Federal case.

JBM Ballistics would suggest that I must be running around maybe 2,700-2,720 fps to get the results I was getting (high hits at 4.1 mils at 600 yards). Could I really gain 80-100 fps with such a small change in case volume?

 

[pell1203]

Re: How much velocity change from brass switch?

In reading through your post above you also mentioned that the Varget powder you were using was from a new can. Curious if the old and new can are from the same batch? Were they manufactured around the same time (i. e., same age)? If not, then there are likely some powder differences that have also been introduced. Your new batch of powder may just be a bit hotter than the old, and this, along with the slightly reduced case volume may be causing an increase in velocity... just something to think about.

 

[Coloradocop]

Re: How much velocity change from brass switch?

These cans of Varget may be from different batches. I know I had two cans from the same batch, and a third can from a different batch (I'd have to look at them to tell for sure).

I've definitely heard of powder from different lots performing differently, but I've never personally experienced this with Varget in the past (though that certainly doesn't mean that it doesn't happen with this powder, like others).

 

[gvanhyning]

Re: How much velocity change from brass switch?

When working up a load, it's not uncommon to go up in .2 of a grain increments and have one shot string that is fine and the next shot string you have some or all the indications of over pressure.

 

[a4308]

Re: How much velocity change from brass switch?

Good info here....