[Richard.Howe]

Hi all,

Quick question about min OAL as it relates to freebore in my rifle, a Browning A-Bolt.

Using a Stoney Point OAL gauge, I found that the OAL required for my bullet to touch the lands is 3.520". Since my magazine will not take bullets longer than 3.370", that's a jump of 0.150" ...but Barnes recommends 0.030"-0.070" for the Triple Shocks. Incidentally, the Minimum OAL for this round is 3.340".

So -- my choice is make my rifle a single-shot for manufacturer's recommendations, or more than double their advised free-travel limits to fit rounds in my magazine.

What is the real-world impact of having this great of a free-travel dimension? Is is typical for other rifles?

Thanks,
Rich H.

[HSMITH]

Rich, I have faced the same problem with a short action gun in 284 Winchester. I wanted big OAL and was VERY limited. The case was worse since I wanted to shoot heavy for caliber bullets.

I don't have a lot of scientific backing, but I started at minimum loads with the max OAL my magazine would take loading the bullet I wanted to shoot. This coincidentally or not happened to be the max OAL listed for the cartridge, but was still about a .075" jump to the rifling. I increased charges until I saw pressure signs which for me were.001" case head expansion coupled with slight primer flattening. In my case I guesstimated a SWAG that those signs meant I was in the 55-60K psi range. I then backed off by 10% and looked at group sizes of the workup loads from there down. I ended up at about 15% under the top load where grouping was good and the velocity was enough for the job.

It worked for me. I ended up with a load that hit what I was shooting at with enough velocity for the bullet to expand well past the distances I was willing to shoot. Be sure to consult the bullet manufacturer for recommendations and guidelines too, quite often a bullet doesn't need near the speed we try to attain for positive performance.

[Jaywalker]

I thought I'd have the same problems with my Ruger M77 MkII in 6.5X55, but it doesn't appear I will. So far, I've been able to seat bullets that both fit in the magazine box and touch the lands; I then back them off a bit. It's this "bit" that's the issue.

Lyman's 47th says (IIRC, it's not in front of me) that many rifles like a jump of from .005 to .020. With this, you'd be within general criteria. Lyman's advises adjusting the jump in .005 increments, as that small amount can make a real difference.

Maybe I'm misreading your post, but it looks to me as if you have plenty of room. From what you wrote, they want more jump, not less. To get to a jump of 0.030, wouldn't you just seat it deeper?

Jaywalker

[swifter]

Jaywalker,
IIRC, the "rule of thumb" is .22(CF)s often like a little jump; .24s & up like to touch the lands. Of course, the other ROT is:" Each rifle is a rule unto itself...":D

Tom

[Richard.Howe]

To be clear, I am saying "jump" to mean the distance between the bullet's starting position in the chamber (before firing) to the point where the ogive begins to make contact with the rifling lands. So, I would seat the bullet deeper to get more jump. I think I've currently have way too much jump.

The problem is that, if I size max OAL such that my bullets just barely fit in the magazine, the OAL is 3.373". My guage tells me that the bullet is touching the lands at an OAL of 3.520".

I therefore get a jump of 0.147". This is pretty big...but is it typical? Is it a problem?

Why would Browning have put the rifling so far away from the throat?

[redneck2]

in the "Gunsmithing" forum if you don't get the answer you want here

This is one of the reasons custom rifles shoot better than factory. We throw around figures of .005 like you measure it with a yardstick. In the real factory world, tolerances aren't held very close, hence the variation in accuracy of various rifles, and the reason some like different brands and weights of bullets, even thought they're "the same".

If you go to a precision shooting site like Benchrest Central, the first thing the 'smiths tell you is to change out the barrel if you want superb accuracy and aren't getting it.

I'd seat them out to max and see how it shoots. I assume it's a hunting rifle, so if it holds 1 1/2", have at it. If not, a gunsmith could re-chamber and set the barrel back

Look at a .357 shooting .38's. They jump more than 1/10", get smashed thru a forcing cone, blown across the cylinder gap, and can still shoot great

[Richard.Howe]

Super point, never thought of that. I might contact Browning just to double-check that these chamber dimensions are within spec.

At any rate, this is a hunting rifle as you suggest. My 1" groups will just have to "do."

Thanks all!

What a great board!

[Jaywalker]

Rich,
Sorry, I ignored the decimal point in your previous post. Let me try again.

You have 150 thousandths jump. Lyman's 47th (pg 97) cites, "A bullet jump of 0.005 to 0.020 will most often deliver the best accuracy." Lyman's is therefore recommending a jump range of 5 to 20 thousandths. By those criteria, and by Barnes' recommended 30 to 70 thousandths, your jump is quite long.

The jump is not unprecedented, however. Some rifle makers make their throats longer than others, and it isn't merely sloppy manufacturing. Weatherby is one such company. The more jump to the lands, the less problem a shooter will have with pressure issues. Avoiding excessive pressure is the reason you don't normally want to have your bullet actually touch the lands, though swifter indicates that isn't always true.

So, I'd agree with redneck2: seat it out as far as you can (since it isn't touching the lands) and see how it shoots. Back it off in stages and see how that shoots.

I have a hankering for a micrometer seating die, but right now I'm adjusting seating depths with fractions of a turn of the seating stem, and trying to approximate a .005 change. (Fourteen threads to an inch is 0.0714 inches, so 0.005 inches is about a twelfth of a turn. Good luck with that for repeatability.)

Anyhow, sorry for the confusion.

Jaywalker

[Mikul]

There is another option a bit more devious: cut the magazine. Seat a bullet to 3.520 and hold it above the magazine. You'll see the diameter of the bullet overlap the front of the magazine by a couple of millimeters. You'll want to draw a center line from the tip of the bullet down and cut an opening that wide plus 1mm centered on that line. I wouldn't cut it much more than 1/2 way down the magazine or the magazine might not hold together too well.

Before you cut anything make sure that a bullet that long will fit in the magazine well. You should also try to simulated a feed from the magazine of a bullet this long. The feed ramp geometry might not allow it.

The best way to correct this is replace it with the proper barrel. Contact Mike Rock or Schneider and they'll chamber and barrel to your specs and you won't have to cut the magazines.

[Steve Smith]

Jump is a funny thing. Some bullets love it, and some hate it. There are no rules of thumb. What's your rifle for? What kind of rifle?

[Richard.Howe]

It's a Browning A-Bolt chambered in .300 Win Mag. Hunting applications only...but being an engineer, you know I have to tweak it...

It was "hunting accurate" before reloading. However, I'm trying to tame the beast down to reasonable whitetail levels while maintaining (or improving) accuracy. Hence my confusion over the HUGE jump dimensions in this gun.

Incidentally, I called Browning today to verify that the chamber dimensions were within factory spec. They were no help at all -- the lady at product customer service had no idea what I was talking about and refused to forward me to someone who did. She said, "why don't you try Browning R&D?" Sheesh...

[Steve Smith]

I'm an engineer too, and I'll say , "no, you don't have to tweak it." I think you ought to be able to find a nice light load without going over mag length. For hunting you always want back-up shots.

[30Cal]

I'd seat them out towards the end of the mag and see how they shoot. For grins, I'd seat some at the Barnes recommendated depth and single feed those to see what the difference is. Might not be a lot. If it doesn't shoot well out of the magazine, then it's time to start looking for a more suitable bullet.

Unless you specify otherwise, the rifling on your barrel (twist and throat) are set on some range of bullets. Bullets usually have a fairly generic profile, but some fall outside of the norm (things like Hornady AMax or VLD bullets). The Barnes TSX's look a little long like a VLD bullet.

Ty