7mm Rem Mag issues...very slow velocity

[buckbrush]

I am wondering if anyone can give me some insight to a 7mm Rem Mag that seems to be having some issues. This is post ’64 Model 70 built in 1968. Every load that has been tried yields velocities about 400 fps slower than most reloading data states it should be going. An example would be a 140 gr Sierra Game King running about 2775 fps. This is with a near max load. The chronograph has been verified with many other guns and seems to be performing well.

The gun was purchased new by my dad many years ago and he swears he ran at most 40 rounds of factory ammo through it. Any ideas what might be going on with this thing?

Thanks.

 

[snowpro440]

how old is the loaded ammo or components ? could it be a bad batch of powder or high moisture content ?

 

[buckbrush]

Fresh loads with multiple powders and primers tried.

Thanks.

 

[bheadboy]

possibly a number of problems

-bbl dia at max, producing minimum pressure thus lower velocity
-long throat, pressure drop before bullet seals
-excessive bbl protective lub, reducing friction
-weak firing pin spring, slow ignition
-excessive head space, case moving on firing pin strike
-sizing die setting shoulder back to far

neck size to firm bolt close, seat bullet in rifling .010, then crono

Bob

 

[buckbrush]

It has a lot of free bore. If I remember right, with 168 vld bullets I couldn't reach the lands and keep the bullet in the neck.

 

[Larryh128]

What primer & powder are you using?

 

[JohnnyJohnson]

Near Max load using what manual? Try Hornady Vol. II.

 

[dmg264]

The barrel hasn't been cut short has it?

 

[buckbrush]

Barrel is 24". I've tried RL22, Retumbo, H-1000, H4831, CCI250's, Federal 215's and bullets ranging from 140 gr to 168 gr.

Powder and primers have worked fine in other guns.

Thanks.

 

[jsn]

If none of the other suggestions has helped so far, I would try three things:

1) Different magnum primer--you might have a weak batch.
2) Slug your bore with a soft lead slug/bullet. Do it from the chamber end. Any "loose" or "tight" patches? Loose early and tight later could be problem. Maybe just too large of a bore diameter to begin with? All three? You won't know until you slug it and measure it.
3) Chamber cast with Cerrosafe. Did your father buy the gun new? If not someone may have modified it, then sold it after being "bubba'd".

Just some thoughts not brought up earlier...

 

[bheadboy]

Th more i think of this problem, i am convinced you have a bore that is oversized, too large a dia. for spec. - you have tried all the normal solutions and found no answer.

lots of reasons this is possible, all conjecture. the answer is to get a smith with ability to check the dia. and replace bbl if that is the problem.

Bob

 

[dennisinaz]

Your barrel is not sealing well, either too large in bore or the chamber is messed up to the point that gasses are getting around the bullet at the beginning. I had a 264 Mag that did this. Every case would be black sooted and partially collapsed. Velocities were 400-500 fps too slow. I was able to do a chamber casting and found the problem. It is a 300 WBY now!

 

[buckbrush]

Would an oversized barrel explain the soot around the holes that the bullets punch through the paper? I've always thought that was weird when I shot this thing.

Thanks.

 

[bheadboy]

if the soot is on the bullet, it is getting past the bullet in the bbl,

check the dia of the bullet, then if it is .284" the bbl must be over .284

either is a bummer

Bob