Short barrels and pressure

[Newtosavage]

A question/comment on the chronograph thread got me wondering...

My 7x57 has a 18.5" barrel, and as I would expect, the velocities I'm reading over my chrono are nowhere near what the published data shows. Usually, I get about 150-200 fps. less than what's shown from a 22 or 24" barrel on the reloading data.

But someone mentioned using velocity to help determine when you're nearing the max load. Is that a thing? If so, it would appear that I would/could load several grains more powder to bring my velocities up, possibly without increasing pressure beyond max. levels.

I'm pretty new to handloading, and I'm sure this is a rookie question, but I'd appreciate any feedback the experienced handloaders could provide.

I'm not trying to turn my 7x57 into a 7mm Magnum by any means, but I would like to get a little closer to the published data if I could, safely.

Thanks.

 

[spitballer]

I'm a firm believer that accuracy is a more important indicator than velocity, but in the case of a single powder using published data I'm inclined to believe that velocity would be a useful tool for estimating pressure. This is an issue that I've been struggling with also, trying to find a suitable powder combination for use with a small case and over-length barrel without incurring excessive pressure. I've found that certain combinations can increase velocity exponentially without increasing overall pressure, but unforunately a certain amount of pressure is needed throughout the length of the barrel to maintain accuracy. Complicating matters is the fact that you can't always believe what you read: in my case I don't believe that proper pressure can be maintained in a longer barrel if the faster powder is burned first, yet this is the impression I got when researching this subject online. Hopefully our resident experts can give you a definitive answer to your question and good luck to you.

 

[jmr40]

If you are using a barrel at or near the test barrel length then velocity is a good indicator of pressure. For example if a load manual shows 47 gr of powder will give 2900 fps from a 24" 308 then 2900 fps from a 24" barrel should be considered a max load. You might reach 2900 fps at only 45.5 or 46 gr in some rifles and load combo's. If so, then that should be considered a max load in that rifle.

If your barrel is shorter than 24" then 2900 fps is probably too much speed and pressure. All of my 308's have 18" or 22" barrels. I expect the 22" guns to be 25-30 fps slower than published numbers and the 18" gun 100-125 fps slower. If you are using a 26-30" barrel then you may well safely reach 3000 fps with the same load.

You also have to consider that no 2 barrels are exactly alike. I've seen over 100 fps difference between 2 rifles with equal length barrels and I've seen 20" guns shoot faster than 22" guns with ammo from the same box.

Another factor is the cartridge. Some such as the 308 lose very little speed as the barrel is cut shorter. Generally the smaller the caliber and the more powder burned the more critical barrel length is. A 7X57 would lose a little more speed than a 308.

All things being equal I'd think a 18.5" 7X57 should be able to come within 150 fps or even less than the published numbers. Especially with lighter bullets. In my experience heavier bullets benefit a bit more from longer barrels.

Powder choice matters too. A common misconception is that you need faster powders for short barrels. The truth is that the same powder that gives the best speed in a long barrel will give the best speed in a shorter barrel. Which usually means slower powders.

 

[theleo]

It's the price of a short barrel. You can mitigate the problem a little with powder selection, but simply adding more powder to make up the velocity is not a wise idea. Could you do it, prossibly. Will you be going over standard pressure for your rifle, absolutely.

 

[BWB]

Like most things in handloading, "it depends".
Keep in mind though that the maximum pressure of any load is achieved in the chamber or within the first inch or two of bullet travel down the barrel.
Velocity loss in short barrels is not due to lower pressure - it is due to the expanding gases having less time to act on the bullet.

 

[Newtosavage]

Thanks for the responses. I was curious if a shorter barrel meant you weren't building pressure as much as a longer barrel, but it sounds like that's not the case.

I'm not at all surprised by the loss in speed. I expected it from the 18.5" barrel. Just wanted to be sure there wasn't something I was missing when it comes to what I may be able to do with hand loads in that barrel.

Incidentally, of all the rifles I've shot over my chrono, only my 20" 7.62x39 Savage bolt action duplicates the published velocities. It's always remarkably close to the data, or what's printed on the box. Everything else I have falls short.

 

[theleo]

Thanks for the responses. I was curious if a shorter barrel meant you weren't building pressure as much as a longer barrel, but it sounds like that's not the case.

I'm not at all surprised by the loss in speed. I expected it from the 18.5" barrel. Just wanted to be sure there wasn't something I was missing when it comes to what I may be able to do with hand loads in that barrel.

Incidentally, of all the rifles I've shot over my chrono, only my 20" 7.62x39 Savage bolt action duplicates the published velocities. It's always remarkably close to the data, or what's printed on the box. Everything else I have falls short.

Slower burn rate powders suffer more in short barrels. Try faster burning powders and you'll they still don't gain the velocity of published data for a 24" tube but they will be closer than the slower magnum type powders.