bbl length versus velocity

[tactical]

Mostly geared toward Carl, but any with info can help.

I have been working with various weapon systems for many years and wonder if other have similar experiences. I have found, with custom made bbls, that the difference between a 22 and a 26" bbl is about 75fps, in 7.62, and with 175 Sierras (Federal Match Load). The 22" will get an average of 2625fps, at muzzle and the 26 will get an average of 2700fps, at the muzzle. This is all under SAC

The M24 (24") will get 2600 fps and the PSS (26) will also get 2600 fps. Since the later are designed to be very good, well the M24 is, why don't these last two get more velocity? Is it the hammer forging, bad tollerences, rough bore or what?

I have noticed the Remington 5R bbls foul like hell so this may be part of it.

Mike Miller

[USMCSNIPERONE]

Tactical, Here we go again ! Carl can answer this better than I ,but here goes anyway.Usually there is approx. 35 f.p.s change in velocity for every inch of barrel that is added or subtracted. There are so many variables to take into account that it blows the mind. the reason you have about the same velocity with your M-24 and PSS is probably because the M-24 has a tighter chamber then the PSS resulting in more pressure which means more velocity ,despite the two inches of barrel differance. VELOCITY THREAD LOOK OUT!!! Sniperone

[tactical]

Sniperone, I did not explain myself well here. I wonder why the 24" M24 can not make as much velocity as a 22" of a custom manufacture.

I have a 26" 5r Remington bbl PSS and it does not make the 2700 that a 26" KxP bbl does. Yes I know PSS's do not normally come with the hammer forged 5R but I knew someone a few years ago that did it in the custom shop for me. They wont put 5R's on PSS's anymore.

Just seems to me that Remington should be able to make a bbl that kept up with the custom guys on the M24.

Mike

[USMCSNIPERONE]

[QUOTE]Originally posted by tactical:
[B]Sniperone, I did not explain myself well here. I wonder why the 24" M24 can not make as much velocity as a 22" of a custom manufacture.


Mike, I don't think there are many production rifle makers out there that have the tolerances that custom barrelmakers hold to.That said and I will catch flak for it! The custom barrels and chambers are almost always tighter therefore creating more pressure than a production barrel,which are built with enough tolerance to accept every type of round you can find,and shoot most of them well. The custom barrels are usually cut to the specifications of the buyer,if he actually knows what he needs! My Highpower rifle is a Colt and after I rebarreled with a Douglas there was noway I could shoot the same load without blowing aprimer. Anyways there are still alot of possible reasons on why you aren't getting the velocity out of the M-24 as the custom barrel,same load?Cut rifling,hammerfogred,larger chamber,ect.ect. Wheres all the experts out there???? Semper Fi Sniperone

[RifleMaster]

Hi Mike!

Interesting MV's! Assuming that you were using the same lot of Match ammo and the testing was done under the same atmospheric conditions (especially nearly the same air temperature), I would guess that freebore/ bullet jump is the reason for these MV's.

I know that freebore length, freebore diameter, throat angle, and the frictional resistance of the bullet entering the rifling will significantly affect MV's. A good known example of this is the need to increase your powder load to obtain the same MV when going to Moly coated bullets. Moly coat reduces the frictional resistance and therefore increases the speed of bullets entering the rifling. This reduces peak case pressure because the case volume containing the expanding gases increases at a faster rate. Of course this is only true if all other case parameters are the same. Longer freebores also cause bullets to accelerate faster which reduce peak case pressures. I believe this effect is greater when slower burning powders are used.

Although I have not tested MV vs. barrel length on one specific rifle and ammo, I believe Sniper1, Jy, and several others are correct that there is about 35 fps change in velocity per inch of barrel length change.

I do not believe barrel fouling is the cause of these MV's. The single point cut Obermeyer 5R barrels that I have used have all performed very well and they were not prone to fouling. They are generally easier to clean.

I hope this helps!

Keep Holding'em Hard!

Carl

[This message has been edited by RifleMaster (edited 03-14-2001).]

[tactical]

Thanks Carl and Sniperone. Carl the bbls are all 5R but the Remingtons are Hammer forged not cut and they foul like hell. The others are either Mike Rock or a KxP varient and smoth non fouling. I had not concidered the lead and think that might be it. The Remingtons have a long throat and all the custom where cut to have a minimum jump with the 175's.

The lots were all the same. A side note is one lot I have tested has run 2750 (26" KxP), under SAC and that seems way fast but shot very well. I am all out of that but the new lots all run well.

Mike