Weird FPS Decline

[Rucker61]

I was at the range shooting the .300 WSM at 300 yrds yesterday, shooting 5 shot strings and letting the barrel cool down after each string. The first 15 rounds were AMAX 168s with 66.5 grains of RL 17, and they averaged around 3170 FPS. The second 15 were Barnes 168 gr TTSX with 67 gr RL 17, and the first 2 strings averaged around 3220-3228. The last string started there, but each subsequent shot lost 15-20 fps down to 3160. All rounds were measured with an electronic scale to one significant digit, so all loads were identical. Any ideas?

[Great-Kazoo]

Battery going on you

[Tim K]

Chronographs are better than guessing, but only just. Might be measurement error.

Or, maybe the barrel was fouling badly. I doubt it, but maybe. Did you notice signs of pressure towards the end?

[Rucker61]

Chronographs are better than guessing, but only just. Might be measurement error.

Or, maybe the barrel was fouling badly. I doubt it, but maybe. Did you notice signs of pressure towards the end?

No, I didn't, and I'll recheck the brass. Bolt opened just like it always had. Barrel wasn't real dirty when I cleaned it afterwards. I'll install a new battery and check again this next weekend.

[Zombie Steve]

Is it possible you got some powder on your scale under the pan? Zero might have changed because of that, or just drifted. I know mine is sensitive and I keep the phone away from it... also any moving air or proximity to florescent lights.


Just guessing. How consistent was that load before? All the same brass? How many times fired? Did POI change at all? Did you run out of one lot of powder and switch to another while loading?


I don't have experience with 300 wsm. At a glance, RL 17 doesn't seem inappropriate for the cartridge, but what kind of load density are you getting?

[Rucker61]

Is it possible you got some powder on your scale under the pan? Zero might have changed because of that, or just drifted. I know mine is sensitive and I keep the phone away from it... also any moving air or proximity to florescent lights.


Just guessing. How consistent was that load before? All the same brass? How many times fired? Did POI change at all? Did you run out of one lot of powder and switch to another while loading?


I don't have experience with 300 wsm. At a glance, RL 17 doesn't seem inappropriate for the cartridge, but what kind of load density are you getting?

All 50 rounds were loaded at the same time using the same bottle of powder. I made sure that the scale remained free of powder under the pan. It's all brand new brass, and I'm running a COAL 15 thousands longer than minimum (Tikka bolt stop mod), so I don't think that load density is an issue. For consistency, for the 15 Barnes rounds I fired, MV was 3228, 3228, 3220, 3228, 3212, 3244, 3228, 3197, 3228, 3236, 3220, 3220, 3197, 3182, 3152. POI did't change appreciably.

Regarding RL 17, from what I've read, it was developed especially for the short magnums. For a 168 grain bullet, I'm a grain under max load.

[Hoser]

I will bet a paycheck on your chronograph is the culprit. Sunlight and sun angle plays havok with chronographs.

[MarkCO]

Heat soak. When the actual bore and chamber are heated from a few rounds, the outer layer of steel has enough strength to arrest any thermal expansion. As the heat goes through the barrel, after enough time/rounds, you are heating up the barrel some. It takes several minutes, and usually 20 rounds or more, to develop, even if you are shooting rapid fire, but it does have an effect, more so on a bore that does not deeply engrave the bullet. Generally, 30 rounds of .308 through a 24" rifle barrel, depending on profile, will grow the length 0.013" or so and increase the bore diameter about 0.0003". Unless you are shooting, and then waiting 20 to 30 minutes, you will have this effect.

If I am shooting and that is an issue for the rifle, or what I am doing, I take a can of airduster. Shoot one, 5 second blast of airduster in the chamber, let it sit for a minute, shoot one... No more than 5 rounds, then a 5 minute cool with a few shots of airblaster. I take another gun to do something else with or I get impatient waiting. You should be able to get 20 or so rounds downrange in a 10 minute period with most rifles and not see this effect. More than that, you need to wait longer or use the airduster if you want cold bore data.

[Zombie Steve]

All 50 rounds were loaded at the same time using the same bottle of powder. I made sure that the scale remained free of powder under the pan. It's all brand new brass, and I'm running a COAL 15 thousands longer than minimum (Tikka bolt stop mod), so I don't think that load density is an issue. For consistency, for the 15 Barnes rounds I fired, MV was 3228, 3228, 3220, 3228, 3212, 3244, 3228, 3197, 3228, 3236, 3220, 3220, 3197, 3182, 3152. POI did't change appreciably.

Regarding RL 17, from what I've read, it was developed especially for the short magnums. For a 168 grain bullet, I'm a grain under max load.

Just like working on a car... eliminate the easy stuff first.


I could see it being the chrono or a hot barrel although I'd lean towards hot barrel. What chrono are we talking about?

The few times my chrono acted goofy, it was obvious. My .44 mag wasn't going 2,100 fps. You're basically changing 2%. If it is your chrono, I've never experienced anything like that.

Last question I can think of... were you shooting at the same target across the screens? In other words, were you shooting at a level target, then shot up or down across the screens at a different target? If so, you're changing the length the bullet flies between the two screens and you'll get slower numbers. A few degrees of angle shouldn't matter much, but could it account for a 2% difference?

[Rucker61]

Chrono is a ProChronoDigital. No real difference in targeting at 30 yards.