What can cause loss in velocity?
Same loads and same power lot. Round count on the barrel is only 300 or so. Somehow I lost about 70-80 fps. I have never cleaned the rifle but it's still shooting amazing groups, just slowed down on me. Confused to say the least

Could be temperature related. Also, you don't mention the cartridge...but its possible you have just a bit of throat erosion causing a drop in starting pressure, and that will cause lower velocity. Another possibility is chronograph error. There could be more possibilities too.

7x300 win mag
H1000 powder

I really don't think it's a chronograph error. I have a 4" steel target at 430 and couldn't get on target. Checked my 100yd zero and it was 3/4" low. Set up a larger target down range and was shooting 4" low at 430. Plugged that info into the GSeven calculator and it showed my velocity had dropped like 85fps. Then I shot it over a chrono and it was reading about 75fps slower that what these loads have been running.

I was reading about one of the top PRS shooters and they were saying velocity loss is just gonna happen. So I guess it's pretty common.

What did you change in the ammo from the first batch to the second batch? Sometimes, when you size differently or have different neck smoothness on once fired brass vs virgin brass, it can change up the fps some.

Also, what bullet are you running? I am building a 7/300 Win Mag right now, and plan to shoot the 195 Berger's with either Retumbo or H1000.

I didn't change anything as far as the brass. I loaded up 200pcs with Virgin Norma 300wm brass and necked it down. I was seeing consistent velocities through the first 125-150 and now this. I am running the 180gr hunting VLD

Rifle gremlins.

What's odd is, my friend has the exact same rifle. Built by the same builder using all of the exact same components. They have always shot extremely similar. The load I have, shot great in both rifles and we have shot the drop data to verify velocity. He was shooting beside me with the same Ammo a week ago and had no issues. It's like it lost the speed out of nowhere. Round count on both rifles is probably within 75 rounds of each other

You didn't push the shoulders back farther than you did for the faster ammo, did you?

I wonder if fouling can slow it down? Though I've never seen that happen.

That's what I'm thinking

I typically take a base to datum every 15 rounds or so when sizing. I'll go back and double check but the brass was all prepped in the same batch

Same powder lot number?

Could simply be difference in barrels. Measure your throat and see. Also run a patch or two down it and find out. Double check scope and chrono too. This is all if absolutely nothing in the load changed.

Yes sir, same powder



I went ahead and did this last night. I really don't make a habit of cleaning my precision rifles but I broke down and cleaned it. I cleaned out all the copper and all. I'm taking it out today to see if cleaning it changed anything.

Was the outside ambient temperature the same as last time you shot it?

For how many rounds was velocity down? Did it stay down for the remainder of the rounds? Wonder if the bass changed lots right there at 150 and maybe case capacity changed, affecting pressures? Just spit-balling here.

I have shot it in summer and winter without significant velocity swings

I probably ran 30 or 40 through it that Day. They all shot consistent and slow

What chronograph are you using?

Did cleaning do anything to it?

Not a thing. I shot it the next day and after a couple foulers I took it to 450 and shot a 5shot 2" group. My buddy pulling Ammo from the same box shot equally well but still 80fps faster. My come up at 450 was 6min while his was 5.25. I quit trying to figure it out and just gonna roll with it. It does pizz me off I have a 7x300 running 7mag speeds

Do you know the round count on you friend's rifle?

within 75 rounds of mine

I think you're seeing the effects of a little throat wear, that's all. You might check your distance to the lands and see if it's changed just so you'll know, and if it has you could try seating a little further out to see what happens.