Low velocity with new 308.

[jmsarron]

I’ve just finished working on my new 308win. My gunsmith has trued the action, installed a maddco barrel and timny trigger on my savage 10 fp with HS precision stock. The barrel is a 26” with a 12 twist.

Soon after starting load development I realized the riffle was producing very high pressure at very low velocity, with moderate powder charges.

I’m using lapua cases, RE 15 and Varget powder, 155 Smk and 155 nosler competition projectiles. I’m getting high pressures with velocities as slow as 2700 fps! even with seating .020” of the lands.

Dose anyone have any input on why this is happening?

 

[SF340Driver]

What "high pressure" indications are you getting?

 

[tammons]

Best to get the problem sorted out before it turns into a major problem.

Could be several things but you might want to check the headspace first. A few years ago I did something really stupid that showed up as pressure.

Inserted a stoney point guide tube in the chamber, filled the barrel with wipe out and laid the rifle down overnight. Of course the chamber gets the foam too. I usually do a finish clean with brake cleaner including the chamber to get everything squeeky clean but for some reason that day I did not. I only did the final clean on the barrel. I ran a big bore mop into the chamber which probably made it worse.

From what I could tell some residue had gelled on the head space surface inside the chamber.

Went to the range with ladders, starting 10% low first round went in fine, but after firing I had a sticky bolt and a extractor mark on the back of the casing. It looked like it was 15% over max when in reality it was 10% low.

The next day I measured that shell and it was way short.

Lesson learned, and a least I did not blow myself up.

There could be other things too, brass at the neck, maybe your barrel does not like that load, bullet jammed, too much neck tension etc but I would check the headspace first.

Best is to buy a go and no-go gage.
Gunsmiths make mistakes too, and I have read of some bad go/no-go gages around too.

If you do not have go and no-go gages and want to do a quick check, put a piece of scotch tape on the back of a new shell and lube it. It should chamber and may be snug but the bolt should close. Put 2 pieces of tape on and the bolt should not close.

Scotch tape is about .002 thick. I would not want to set the headspace like that but it would give you a quick indication.

I would measure the pressure sign brass length too.

I am no expert on this stuff. These are just some things I have run into, so maybe this will help.

 

[SF340Driver]

The reason I asked is that there may be nothing wrong at all. If all you see is a flat primer it could be too low of a charge -- keep working up SLOWLY. If you see ejector imprints or a stiff bolt don't go higher.