Here's a good one for you super smart shooters- longer barrel & lower velocity?

[savageboy]

I shoot .260AI in 1000yd F class. I was running a 26" bbl last year- Shilen 1:8 twist and got 2925ish fps w/ the Lapua 139's at the beginning of pressure signs. I used Rem brass sorted last season and was able to use 48.2gr. of H4831SC seated +5thou into the lands (that is what it liked best).
This season I went to a 28" barrel - 2" longer- Same barrel mfg- Shilen Sel. match, same twist, etc, used new brass (Lapua 308 necked down and blown out brass) for this season. At about the same looking pressure signs seated at -10 thou (what it liked best accuracy wise) could only get to 47.5gr of the same powder (the brass appears to have less capacity vs. the Rem) and could only get 2835fps. Longer barrel, same looking pressure, same bullet almost 100fps LESS velocity.
Any thoughts as to why? I was anticipating an increase of 40fps or so w/ a longer barrel- same twist, mfg. etc.

 

[deadlyswift]

I am interested to see what some of the explanations are on this. I am with you I would have thought there would have been an increase in velocity as well. Maybe with less powder in a longer barrel your powder is burning out to quick and the drag or friction with the bullet still in the barrel is where your losing the 100fps?

 

[Chuckhunter]

If that had happened to me, the first thing I would have done, is load up that Remington brass with your old load and chronograph it,through the new barrel. Keeping everything exactly the same as the original load and noting any difference in temp. If at that time the barrel shot the same or a slower velocity, I would assume that this particular barrel was just slower. Sometimes when running a powder charge that is close to the limit, a .015 change in seating depth can have a dramatic affect. Especially when going from contact to off the lands.

 

[fivering]

You mentioned one of the factors already, less powder capacity in the cartridge = less gas to propel the bullet.

If the first barrel you mentioned, with the higher velocity, was replaced by the shorter one, the first barrel probably had a lot of rounds down it, so it was internally a little larger, more rounded lands, ie: less internal friction. The new barrel will have sharper shoulders on the lands, cutting into the bullet more.

The new barrel ID is probably a few ten thousands difference from the old one, giving different friction amounts.

Even though the barrel is from the same manufaturer, they may have used a different reamer, giving you a slightly different chamber capacity.

When strictly determining the effect of barrel length on velocity, you will notice they start out with a given barrel and measure velocities after cutting down the SAME BARREL in even increments. This removes the variables mentioned above, and others as well.

 

[savageboy]

The weird thing is that the shorter barrel gave over 100fps more vel than the longer one. I was going to try the Rem brass next. Most of it is hammered though. That was the next thing, just curious now. I am going to use the Lapua- bought and prepped 300 new ones for this season, so I have alot of work into it. Making 260Ai from .308 is a bit of work. Size it down in 2 steps (7mm then 6.5), fire form, trim to length, clean up the necks, chamfer,etc. Wow.
I did fire 5 rounds w/ a +5thou jam w/ the same powder charge and gained about 40fps compared to the 10 thou. jump. FYI. The 10 thou jump shot better in this barrel though for accuracy.

 

[Bast]

I agree with Gunamonth, 2 identical barrels won't react the same. Your new barrel is longer so this is a big difference. Also to increase the speed, use a longer jump (.020) but you may loose some precision.
Longer barrel = slower powder
You may try a different primer first.

 

[cmillard]

i had the same problem when switching brass. that is why, whenever you switch a component--bullet, primer, powder, brass, etc--you should start your load development over, or at least take a few steps back in powder. i was using winchester weight sorted brass for my ar-15, then got ahold of a bunch of military once fired federal brass. this once fired military brass was HEAVY and the case capacity was small. even with a mild load (mild in the winchester brass), it would pop the primers out of the case. so now, i just stick with the winchester and Lake City military brass. this is why i ALWAYS sort my brass--even a moderate load in brass that has a smaller internal volume than the brass that the load was developed with, can create severe consequences.

 

[JRinCo]

I've been shooting a standard .260 Rem for 8 years and started using Lapua brass when I got my first barrel, a couple of years later I decided to try some Remington brass. I discovered the same thing that you have, Rem. brass has a little more case capacity than does Lapua and you can get about 100 fps more from Rem. brass. I found no difference in accuracy but the Remy doesn't last nearly as long. Reloader 17 will return your horse power.

 

[savageboy]

Thx for the confirmation. I was going to try the RL17. I have an 8lb jug of H4831sc though.