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Fast versus Slow Barrels

20 May 2018
@ 02:31 pm (GMT)

Luke Lahdenranta

I was just wondering about what factors cause a barrel to be fast or slow? We know that if we take any otherwise identical rifles in 30-06, let's say Rem 700s a half a dozen of them could easily vary 75-100 fps shooting the same load either from same lot# or hand load. What i am wondering is if we know what exact dimensional variances could cause a barrel to run either fast or slow ? So let's say if we measure all the barrels we may find the faster barrels to run let's say .3075 instead of .308 and generate a little more pressure so run faster, am I thinking about that correctly? Tighter equals more pressure equals more speed for a given load? Or can it be thought of the other way, that for example my long throated Tikka 30-06 I can seat bullets out for more powder capacity fir more velocity generation? I was just chronograph loads for my 30-06 with Superformance powder and 200 gr Sierra game kings averaging 2720 fps, and pretty accurate at sub MOA. I am pretty pleased with that. But yet factory 180s will barely run 2600 fps.

So, has anyone else pondered the magic that is faster or slower rifle barrels?


20 May 2018
@ 03:13 pm (GMT)

Paul Leverman

Re: Fast versus Slow Barrels
Some thoughts that aren't based on anything but conjecture and guessing. The micro-finish probably has much to do with it. If there was a coarser surface finish, then more friction would be induced. If a bore was tight already, then with the additional loss of energy trying to overcome the resistance to movement, velocity loss may be substantial. Angle of attack at the leade may be another factor. With a gentle, easily engaged angle, the bullet will start to rotate slowly upon initial engagement with little loss in forward momentum. If the angle is sharper, when the bullet engages the rifling, forward momentum may be slowed until the velocity overcomes the resistance. As to the relative slowness of factory ammo (your 180s, for example), they have to be loaded to be safe in all actions and all barrels. They aren't so much as downloaded as that they are not as "hot" as they could be. This is probably a factory safeguard against litigation in case someones Plasticine 500 Magnum blows their face off. It is also very unlikely, but I could be wrong, that factory ammo uses magnum primers. The exception to this would be the hugely overbore cartridges that require a great deal of heat and length of flame to set of the hugely long powder columns.
Other than that, I'm out of ideas.
21 May 2018
@ 08:49 am (GMT)

Nathan Foster

Re: Fast versus Slow Barrels
Hi Luke, yes there are a number of factors, not just within the bore section of the barrel, but also and especially within the chamber. A loose chamber (case body section and or throat section) can cause a drop in velocity with factory ammo, but it can also allow more room to develop power with hand loads.

There is certainly a lot to it and as Paul alludes to the theory of 'should' does not always pan out in reality. A smooth bore should for example decrease friction. But if the bore is too smooth, it increases resistance. This seems to be a contradiction in terms and it is, but only because the words we use in loose discussion are not scientifically accurate in their descriptions.
22 May 2018
@ 12:13 pm (GMT)

Luke Lahdenranta

Re: Fast versus Slow Barrels
I can definitely see how the finish inside the barrel can have a major effect on velocity. I have a Marlin 30-30 with a poorly finished 'worm skin' bore and I am planning on running some loads through the chronograph next time out now that I have one. I will be interested to see if it will run slow as I expect since it looks like a copper mine inside after a dozen rounds. I also have an older Marlin 32 Special with a tight bore that mics at 0.3195 or 0.320. I am interested in seeing what speeds this runs with the Hornady Leverevolution, it shoots them extremely well.

I also had an opportunity to run a dozen rounds of the federal fusion 95 gr 243 load through my Tikka and that ran at only 2850 fps avg versus 2980 fps on the box. This load has the boron nitride bullet coating so I am wondering if that coating is reducing pressure enough to lower the velocity significantly. I also ran a dozen rounds of Winchester deer season XP 95 great which ran equally slow as well at 2985 avg versus 3100 on the box. I would like to retest after cleaning that boron nitride out of my barrel to see if that is leaving a residue in my barrel and effecting velocity.
23 May 2018
@ 10:13 am (GMT)

Nathan Foster

Re: Fast versus Slow Barrels
Just for comparisons Luke, in the Ruger American .243 test rifle I have here, the average velocity with Winchester Extreme point is 2900fps which is about right for a 22" barrel, losing 35fps per inch from the test barrel length.

As an added FYI, the wound looks much the same as the .223 V-Max, large entry into the onside ribs, fist size internal wound, then tapering rapidy. Exit wounds are either non existant or very small or up to two or three pin hole exits from fragments. The pill is frangible but more so than others due to the wider HP behind the tip. It is well designed for the average johnny with a plastic pop gun, aiming well behind the shoulder, no major bone. In other words, the increased wounding potential of this remake of the Silvertip helps overcome both rifle and shooter errors but at the expense of penetration. It is therefore important to match the bullet carefully to game weights. The .243 load is OK for lighter game, most emphatic on weights below 130lb.
25 May 2018
@ 04:33 am (GMT)

Alvaro Piqueras

Re: Fast versus Slow Barrels
Hi Luke

I found 2600fps as the “standard” for 180gr factory loads; even used in auto loaders. I have tested some ammo brands over mi RCBS chrono (not as much as Nathan for sure) with different rifles (mostly sightin in friend’s rifles).

What I’m tryin to say is that maybe your tikka is not a fast one... i will take a look at my handloading data for a really worn-out (long-throated) 3006 rebarreled mauser 98. If I recall well i went well passed book max lods and getting hugh speeds without step into high pressures (I mean dangerous pressures).

As a side note, I have found high pressure signs with superformance ammo, with full-flattened primers that render brass mostly non-reloadble...

Hope this help you to get a better picture.


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