Does longer freebore affect speed and pressure?

[NamibHunter]

For new (-ish) barrels: Does longer freebore allow more powder and more speed if you load to a small jump?

Say a long bullet is inserted only 0.2” into the neck, either because the reamer cut a slightly longer freebore, or the lands have eroded some amount. Effectively case volume goes up a lot. Does this help us achieve more speed by adding more powder, without running into pressure problems?

Example below: 153 gn Atip projectile, which is very long at 1.5”, in 6.5 Creedmoor brass.

 

[supercorndogs]

Yes.

 

[NamibHunter]

Thanks. It looks like i can add 2 grain of additional powder as a result. Lands have moved forward by 0.2”. Does this make sense?

 

[Supersubes]

Yes technically, but it doesn’t have as great an affect as you’d think. On two occasions I’ve chambered a barrel, developed a load, then ran the throat out further with little affect. In one case I added .138 to the existing .112 throat. It gained roughly 30 FPS. You could see a greater variation across two different barrels with identical specs.

Pick the throat on the practical aspects like mag feed, appropriate sized action for the cartridge as opposed to using a longer action, etc.

 

[Supersubes]

Thanks. It looks like i can add 2 grain of additional powder as a result. Lands have moved forward by 0.2”. Does this make sense?

What you’re describing would be the same as comparing capacity between a 6.5x47 and a 6.5 Creedmoor, or a Creedmoor to a .260. They’re performing almost identically in a lot of guns in hand loaded configuration. If you want to go faster, pick a larger cartridge and/or a longer barrel.

 

[Milo 2.5]

By seating bullets that far out, you end up with not enough of the bullets bearing surface in the neck, causes other issues. If you are chasing the lands on a barrel on it's last legs, fine for awhile. I would never shoot that look in a new chamber.

 

[NamibHunter]

Good feedback, thanks much. Yes it is now loaded way beyond mag length. Single feed only. [On my new MPA rifle with a fresh barrel, the bolt will not be able to close, loaded this long.]

So to summarize, and correct me if i am wrong: Longer freebore in a new barrel might perhaps gain you 30-50 fps, but it is not going to turn a 6.5 Creed into a 6.5x284 or a 6.5 PRC. A barrel with a slightly larger than SAAMI spec chamber might do the same, i presume.

Case closed, and thanks for the details!

Time to build a 6.5 PRC! My MPA rifle is a switch lug gun, so it is as “simple” as buying a new barrel, bolt, dies and brass and go/no go gauges...

Not really cheap to change caliber, but a lot of fun!

 

[NamibHunter]

What you’re describing would be the same as comparing capacity between a 6.5x47 and a 6.5 Creedmoor, or a Creedmoor to a .260. They’re performing almost identically in a lot of guns in hand loaded configuration. If you want to go faster, pick a larger cartridge and/or a longer barrel.

Just curious: Is it worth it to rechamber an existing 6.5 CM barrel in 6.5 PRC, which should remove the eroded lands in the process, in order to get 200 fps of extra speed, if the barrel is halfway through its expected 2,500 expected life span? Want to shoot the 147/153/156 class bullets close to 3,000 fps. Or bite the bullet and buy a new barrel?
I have two barrels to choose from, and three actions i can mount it on. One barrel is a 30” Shilen with perhaps a 1000 rounds on it. Shoots 0.6” groups in the Savage. One of the actions is a Curtis Axiom switchlug mounted in an MPA chassis that can feed rounds with a COAL of 2.9”, and this feels like a more viable option.

Second action is a Remington 700 with a factory barrel that shoots bad groups over an inch at 100.... It might well need blueprinting and squaring of its lugs, and not sure that will pay off.

Third action is a Savage 12 Large Shank action with a longer than standard mag that currently shoots 0.5” groups.

Advice would be appreciated.

BTW: I know it is not advisable to rechamber a shot-out factory barrel, as it probably won’t last long even if the new chamber is longer and removed the eroded section of the lands. Waste of money then.

W

 

[NamibHunter]

Just an update: Shot my personal best groups ever at 600 this past week with that “shot out” barrel (3,500 rounds down the tube) using 147 ELDM’s. Fired three groups in quick succession, while watching the wind flag, each a three shot group. Factory Savage 12, factory barrel, no muzzle brake, no tuner, “Target Accutrigger” set to around 1 lbs, Rock BR front rest, Vortex Razor scope at 16x, big heavy sand bag at the rear, off a sturdy bench. Highly optimized 147 ELDM/H4350 load in Lapua neck turned brass. Mid-week so nobody else at the 600 yard line.... they must all have seen the weather prediction! It was so satisfying to stack them on top of each other and color in the original splash mark on the steel!

Hurricane Laura (cat 4) was in the gulf about 8 hours away from making landfall and only 300 miles from us, and wind speed was high (maybe 14 mph). Stiff flags. But very very constant wind speed and mostly constant wind direction, but not enough wind to bump the shooter around. Must be because there was such a strong low pressure center in the Gulf of Mexico? First three groups were all below 0.2 MOA! Very happy with that. Any of the three groups would have qualified as my new personal best.

But: Eventually the lucky streak has to end - Group 4 was on the way to be around 0.15 MOA when suddenly wind direction shifted 30 degrees, which i did not notice (forgot to check the flag), and the group became a 0.8 MOA group. Oh well.

Conclusions:
1) Clearly i still suck at wind reading, but at least the gun likes the load. But slowly getting better...
2) Hurricanes are your friend... as long as you are 300 miles away. Never seen such a consistent wind condition.
3) And yes: Sometimes an old shot-out barrel with heavy carbon buildup can be rescued and made to shoot again... and you van enjoy it for a little while longer.
4) Switching to a long heavy bullet and redoing load development made a big difference here, even though lands have moved probably 0.2”. [I still wonder how many barrels are retired too early.... But to be honest: I am very surprised the old barrel is still performing! It might be dead soon, but not yet!]

Next time there is a hurricane, maybe go out and try to beat your personal best.

 

[NamibHunter]

What a difference the wind can make!

Same gun, same ammo, same load shot a 4” group and a 7” group at 600 yards yesterday. Wind was gusting between 8 and 16 mhp, and 30 degree direction changes.

Recorded SD for both groups, at 5.6 and 5.9 fps, Eas at 12 and 15. Which is very good. So not an ammo issue.

I have to get 10x better at wind reading and obsess 10x less about ammo quality!