Unexpected high velocities from slow-twist rebarrel?

[RedDust]

I've been lurking for far too long - time for a post!

I've recently had my marlin 336 re-barreled.
It's a dedicated cast-only rifle used for target competition and hunting - It had about 2000 rounds through it last year.
The original 20inch, 1 in 10" twist microgroove in 30-30 was replaced with a 26", 1 in 14" twist hand-lapped chrome-moly barrel in 30-30, with a Marlin 30-30 profile.

Velocities seem to have increased a lot more than I expected. Just wondering if the pressures might be up a bit in the new barrel, as 300fps increase in velocity from a 6 inch longer barrel seems rather a lot of difference. Anything to be concerned about when working up conservative cast loads from minimums in the loading data?


------------------------------------------------------------------------------------
Background:

The Cast Bullet Engineering 313-112GC (124gr lubed and gas-checked, sized to.311) over 20.5 of H4198 came out of the old barrel at 1700fps, the same load shoots more accurately out of the new barrel and comes out the muzzle at 2025fps, a 325fps increase.

I can't shoot Lees TL314-90-SWC, sized .311, slow enough using TrailBoss - 5.5gr gives 1275fps and 6.5gr gives 1415fps - big groups at these velocities - I'll need a heavier projectile for plinking.

I haven't tried any heavier cast yet as my NOE 311-165 RDs are too fat in the nose for the new bore. Just got an NOE 311-180 FN mold (311041 copy) so aiming to have some projectiles to test next week. I want to work up to the 28gr H4895 load with the #311041 clone.

I know that the chamber in the new barrel is a little tighter than the original. About 50% of the old neck-sized cases had some resistance chambering, so about 30 remaining hand-loads were pulled, and all cases full length sized. The bore is smaller, but on the other hand there is much more groove area and it's shiny-shiny, not dull like the old barrel.

[Larry Gibson]

Depending on powder and cast bullet used 320 fps, while seemingly a large amount, could simply be the difference between the 20" and 26" barrels. Also the load 's time/pressure curve will have a greater influence in the longer barrel. There is/was a lot of internal difference between those two barrels.

Try 2.8 gr of Bullseye under your TL314-90-SWC for plinking. No wad or filler needed. Velocity will be 900 fps +/-.

Larry Gibson

[Ballistics in Scotland]

It isn't impossible to get that much of an increase, but I think unlikely, and it would surely be with a slow powder. I can't see 4198 doing that.

Could you be shooting with the muzzle six inches closer to the chronograph? That might cause the chronograph to register the velocity of the gases, which accelerate momentarily to a higher velocity than the bullet.

This photograph, taken by a Professor Boys in a corridor of Imperial College in 1893, uses no camera or lens, only the shadow cast by an almost inconceivably brief electric spark on a photographic plate. It isn't at the muzzle, so it doesn't show the outburst of gas there. But it does show how a pressure wave and eddies in the air can cast shadows. You can also see swirling in the shadow of a sunlit glass of whisky when you pour water into it, or even one of those colourless things if you will make that sacrifice to rule out colour as a variable.

[Texas by God]

I 've seen 150fps difference between two identical .243 Rem 700s with 22" barrels using the same ammo. There are slow barrels and fast barrels with no explanation that holds water.
Best, Thomas.

[5Shot]

Same bore dimensions or is the new barrel tighter? Are you sizing your bullets accordingly?

[Tackleberry41]

6 in of barrel is alot of difference. I lost 400fps cutting a chunk off a mosin barrel down to 17". I have used 4198, I get all of that 400fps back in a mosin with a factory length barrel using the same ammo.

[Ballistics in Scotland]

...no explanation that holds water.
Best, Thomas.

Yes, that is a factor many forget with rifles. I would expect two modestly loaded .30-30s with a fast powder to produce less difference in velocity than two .243s, though.

[Scharfschuetze]

I have short barrels that shoot faster than long barrels in the same calibre with the same loads. Who knows why, but some barrels are significantly faster than others, often of the same brand. Internal finish, dimensions, chamber, throat length and diameter... It all plays a part I guess.

The only real way to determine how much you gain or loose by length with a specific barrel is to start your test with a long barrel using the same loads. Chronograph enough rounds to be a valid measure or average of velocity and then cut the barrel back an inch at a time and repeat. It will tell you what is valid for that barrel only. Unfortunately, you loose your barrel doing it.

Some barrels are accurate and some are fast. Hopefully your barrel is both fast and accurate. I'd just consider your new "fast" barrel as a gift from the shooting Gods.

[Jniedbalski]

Larry is right about the 2.8 grain load of bullseye . In my 308 that load to 3.2 gr is awesome with the lighter bullets. At 25 to 50 yards there just as accurate as a good 22

[RedDust]

Thank you all for the responses.

[Dryball]

didn't the Houston Warehouse Project address this? I know they tested barrel length as it relates to accuracy, but, IIRC they touched on velocity as well.