Affect of freebore on velocity?

[Crow]

Had the opportunity to shoot my FTR rifle quite a bit this weekend after getting the barrel back from Mike Ezell (Gunsandgunsmithing on this forum). Mike does really nice work, and I couldn't be more pleased.

Mike installed one of his tuners and took my freebore out another .060 (to about .190, based on chamber cast measurements). This leaves the boattail/bearing surface junction of a B200.20X about halfway down the neck, which doesn't appear to even slightly compress 43.9g of Varget w/.010 jump, resulting in much more consistent seating depth than before.

The barrel now has 513 rounds down it. Prior to extending the freebore, 43.9 Varget was producing about 2660 fps (Oehler 35P), which matched Berger drop table calculations pretty closely. I haven't chrono'd the barrel since the freebore was extended, but long story short, all other things being identical (namely, 31.5 barrel length, B200.20X, 43.9 Varget, .010 jump), working back into velocity via drop charts indicates I'm only getting around 2600 fps now.

Since getting the barrel back, I've put 42 rds @ 100 yds (playing with the tuner), 15 rds at 500, and 73 rds @ 600. The 500yd & 600yd tests both indicate I'm getting about 2600 fps, if you can trust physics and the Berger drop chart calculator.

I know this is getting rather long, but I was surprised to see that a .060 increase in freebore resulted in a loss of ~60 fps. Is this typical?

(Edited for clarity)

 

[dkhunt14]

Now you have more capacity. It will take more powder to get back where you were. It could have changed the load tune also. I have seen where .050 in a WSM was close to 2 grains change. Matt

 

[Crow]

Thanks Matt.

I didn't lose any precision with the freebore change in calm conditions, although the tuner may be offsetting that.

Sort of begs the question of if the additional freebore was worth the effort. Given equal precision in calm conditions, the faster load would be the better choice on a windy day.

Only one thing to do, I guess... see if I can find a good tune > 43.9gr that gets the velocity back up there.

 

[Richard Coody]

that is how roy weatherby made his beans. give that bullet a running.start.

 

[Homerange]

that is how roy weatherby made his beans. give that bullet a running.start.

And what the RUM's are based on too.

 

[270WinDude]

that is how roy weatherby made his beans. give that bullet a running.start.

Yup. longer freebore will gain velocity however in this guys case it sounds like the pressure difference created from seating depth change outweighed the jump. We can only speculate though until he chronies it with the new load. My guess is the math is off.

 

[Richard Coody]

what we use longer freebore these days is to get those long, heavy for caliber, high bc vld style bullets further out in the case. case volume increases and pressure drops. add powder.

 

[Crow]

Hoping to get the chronograph out this coming weekend. That'll pin down the actual velocity, but I'm thinking it'll be on the low side of 2600 fps. I'll post that number once I have it.

Also, realized I'd made a misstatement in my original post: that I have half the bearing surface of a B200.20X in the neck. What I meant to say is I have the boattail/bearing surface junction about halfway down the neck.

Thanks everyone who's commented so far.

 

[billlarson]

YES.........
bill

 

[jlow]

You also have less neck to hold the bullet so with the same “neck tension”, you are going to have an earlier release and less pressure build up, so lower MV. But surely most of it is due to increase in case volume, less pressure for the same load.

 

[Crow]

Thanks jlow - I plan to try another .001 of neck tension.

I still remain completely amazed by the idea that another 6/100 of an inch of room could have such a dramatic effect on pressure/velocity, but others have seen it too.

 

[jlow]

The reality is with precision rifle reloading a 60 thousands change in seating depth is a HUGE change. I honestly doubt you could make up for the drop in volume with an added one thousands change in neck tension. Best try to increase the load if you want the MV back.

 

[Crow]

Yeah, definitely gonna do that too. Weather looks good for this weekend's chronograph session...

 

[Crow]

Well, got out to the range with the chronograph yesterday morning - too early for ideal light conditions, but I proceeded anyway since my main interest was getting 'ballpark' numbers using small samples (5 rds each). The chrono complained, and threw a few oddball numbers which I threw out, but think it answered some questions.

43.9 Varget loaded to the same CBTO as before the freebore extension (.070 jump) produced 2633 fps, about 25 fps slower than before.

43.9 Varget loaded for the same jump (.010) with the extended freebore produced 2658 fps, almost identical to the original load.

Since I was anticipating a need for more velocity when I started this exercise, I also ran a variety of loads between 44.4 and 45.0 Varget. I reached noticeable compression at 44.9, and was able to achieve 2700 fps before I was done, but precision suffered. I was watching the brass closely, and none of it seemed any worse for wear than the 43.9 loads.

I need to go back and re-test under better light conditions, but was sort of surprised how closely the 43.9 Varget/.010 jump load matched my previous chrono numbers. And as I mentioned before, the results on target are virtually identical.

I'm absolutely certain of the amount of elevation I was using on the scope before the freebore extension (it's a number I write down after every trip to the range, along with windage), so I've had to come up with another theory on why I had to dial in over another 1.75 minutes of elevation to get back on target: the addition of the tuner and the attendant barrel re-installation must've had something to do with it. Having trouble figuring out why that would be though....

Thanks to all who jumped in to help. If Gen. George Patton had been an avid F-class shooter, he might have said "I believe Accurate Shooter is the greatest shooter's resource ever devised".

(Edited to correct paragraph formatting issues.)

 

[dkhunt14]

I would try to put the bullets in the lands .005 and try the faster load or load test. One of the reasons for the bad group at higher velocity could be the tuner tuned for the slower load. The addition of adding the tuner weight would change impact at 600. Matt