longer freebore = how much,if any) extra velocity?

[memilanuk]

Anybody have observed numbers for difference in muzzle velocity between one freebore length and another?

I'm looking at picking up a pre-threaded & chambered 6mm BR barrel for the wife's F-Class rig - I'm shooting a Dasher right now, but I still have all the plain 6 BR dies, headspace gauges, etc. Most of the available PTC barrels in 6mm BR seems to have a 0.060-0.080' throat, regardless of whether they have a 14, 10, or 8 twist.

I was considering ordering one anyway, knowing it'll have a short-ish freebore, seeing what kind of MV I can get with it using 105-108 bullets, and if its still a little low, taking it to a local 'smith who has a 6mm throating reamer and pushing the throat out to 0.110' or so and seeing what that gains me. Kind of a non-reversible process,short of lopping a bit off the barel and re-chambering) though, so I'm kinda curious what sort of numbers other people may have seen between short and long throated barrels in the past. I realize that there are a bundle of other variables involved; just curious at this point. I can run numbers in QuickLoad to tell me what it might be just based on the extra volume that it would create, but its still just a 'model', not real-world results.

Anywho, thanks in advance to anybody able to push some info my way.

Monte

 

[sparker]

Hi Monte. My first BRX chamber had like .160' freebore. That was probably too much. Given the .100' more neck with a straight BR, .080 to .100 should be good. The old rule of thumb was have one caliber length of shank in the neck if possible. I know from my experience that better concentricity comes with that old rule. Make up a dummy cartridge and send to Kiff.

Scott Parker

 

[js223]

The Mcgowen Prefit 8 twists have a long freebore. Someone posted on here that it was .113', I think the reamer is a PTG.

 

[memilanuk]

McGowen lists two 6mm BR reamers... the one I was interested in specifically was the one used for barrels stocked by a specific vendor... which most people *think* uses a 0.060' freebore. The other one has a 0.110 or so freebore... but then I have to wait for it from McGowen,6-8wks), and might as well send in some dummy rounds in the first place.

Back to the original question: any difference measured between the two.

 

[dreever]

Monte,

I have two 6BR's with .269 necks. One with an .060 free-bore the other with a .104.
Both rifles using identical loads of 30.5 grains of Varget/CCI 450's/ Lapua brass/ Berger 105 VLD's and Berger 108 BT's respectfully run almost identical velocities of 2940 fps, with E.S.'s in the single digits. Both rifles have very similar accuracy with best groups @ 600 yards under an inch. If I had to give an edge to one or the other it would go to the .060 which seems to shoot just a tiny bit tighter. Both barrels are at the 1000 round mark, and I have had to chase the rifling a bit. Using 105 VLD's in the 6BR with MODERATE loads I see no advantage in going with a longer than .060 free-bore. If you are hell bent on maximum velocity with brass wrecking pressures well then yes a longer free-bore would probably serve you better. Right now I have 22 loadings on some of my brass and it still is shooting flawlessly.

Danny

 

[memilanuk]

Thanks to those who have replied, both on board and off.

One of the suppliers for pre-threaded & chambered barrels for Savages got a stack of Brux barrels in, so after talking with him a bit I think I'm going to make up a couple dummy rounds and send them in. Going with a 28' Savage Varmint contour,tapers from 1.062' @ the breech to ~0.800' at the muzzle). Longer wait, but probably worthwhile in the end.

It is kinda interesting to see the velocities cited for various dimensions - some of which are awful close to what I get out of a 6 Dasher with three grains more powder!

Thanks,

Monte

 

[norpacmiami]

Lynn,

What powder r u using? That's pretty good speed. I have a rifle with really short leade, and 29.5 grns of Varget is already past the limit.

On another 6br, I have a .130 leade, and 30.0 grns of Varget is the accuracy node, with no pressure signs whatsoever.

Andy

 

[Moderator]

Andy
I am using Varget powder and have for about 10 years now.30.3 grains gives me about 2850 fps and the 31.5 gives me 2970 fps.
Lynn

Just to give you guys some ideas of differences in barrels and freebore. My freebore is about .090'. The barrel is a PacNor, a tight 0.236 bore.

30.3 Varget gives me about 2910 fps

30.8 got me to 2950 fps but it was too hot, showing bad ejector marks and there was a stiff bolt lift. I consider 30.6 practical max in this barrel. But I do know there are 'slower' lots of Varget than mine.

And bear in mind that you could change these FPS numbers +/- 40 fps just by changing chronographs.

 

[norpacmiami]

Touche' on chronographs propensity to give different readings, one from the other. Seen it happen one time too many.

I usually don't push the 6br too hard. It's accuracy at around 2,800fps is amazingly enough. Same thing at 2,650, good for 300M.

I have tested different powder/primer combinations, with great results, but always end up returning to good old faithfull, Varget. I just don't like surprises, you know......

As to freebores, anyone has played with long leades and Berger 115 VLDs in a 1/8' twist barrel? I guess it would be interesting experimenting with this scenario.

Andy

 

[dreever]

I think to many people get tied up in chronograph numbers. I hear to many guys complaining that they are shooting bug holes but want more velocity because they are running 2800ish from their barrel and 'It just isn't enough'. For some reason a lot of guys have it in their heads that the closer they get to the magic,in their minds) 3000 fps., the better off they'll be. My answer is 'If it's shooting bug holes, why not leave it alone?'. No two chronographs are alike some can be 50-60fps different than another. I use a chronograph to tweak already accurate existing loads with the goal primarily to be to lower the Extreme Spread numbers into the single digits. Low E.S. equals less vertical, and that's what I'm looking for. Once there the chronograph has served it's purpose for me. My Beta Chrony isn't an Ohler,sp) 33, but for what I use it for it dosen't have to be. I don't drag it along every time I go to the range, heck it only comes out when I put on a new barrel, or when accuracy is falling off. You don't need brass wrecking velocities from the 6BR to be competitive, along with the hard extraction, ejector marks, etc. that goes along with it. In fact rarely are the top velocities as accurate as the more sane speeds.

Danny