Loading for a short barrel

[mcoliver]

Excuse the noob question.

If have a load that shoots a certain PF using a 5" barrel, I'd expect lower readings when the same ammo is shot on a shorter barrel, right?

If I look at at it the other way around: A load that meets a certain PF for a short barreled gun, is there a risk of over-pressure when shot on a regular 5" barrel?

Thanks.

[Matthew_Mink]

I don't think so, pressure should be the same. The higher velocity is a result of the longer barrel, not higher pressure.

[BSeevers]

MMM I would caution to check load out before running it. There is an article in American Rifleman this month about 223 remington vs 5.56 military. They are NOT the same "standards" and neither are different pistol barrels, even in the same caliber. Chambers do determine pressure and one companies may vary even in the same model. Now would it be dangerous or out of control? I doubt it very much. Could it be higher velocity and pressure? You bet it could.

Fitting/reaming/amount of holes/brass make/etc. effect this equation also.

[Zak Smith]

I was thinking about this question earlier this week in the context of rifle loads:

Imagine a max load for 308 from a 24" barrel.  Assume that
going to a faster *or* a slower powder will yield a slower velocity   
(at 24"), when loaded to max pressure.  The maximum pressure will   
happen when the bullet is within 6" - 10" of the breech.  As long as
the barrel is long enough to achieve the max pressure (in that 6-10"
range -- call it 12" for margin), after which pressure trails off, all
you will lose to a shorter barrel (e.g. in the 12-20" range) will be
the incremental loss of velocity in that low-pressure regime.

see http://www.shootingsoftware.com/pressure.htm  for some graphs

I claim that by going to a faster powder in a shorter barrel, all you
can do is reduce blast while losing some velocity vs. the slower "max"
load.  By definition, the fast powder will peak and trail quicker.

I downloaded the demo of an internal ballistics program QuickLoad
http://www.neconos.com/details3.htm
and played with a few things.   Using a basis of a 300WM rifle with
two powders "Quickload demo fast rifle" and "Quickload demo slow
rifle" which are obfuscated so we'll buy the software obviously, I did
the following experiment.  Start with a 24" bbl, and chop it down by
5" every time, noting the max pressure and the MV:

73gr QL Demo Powder "slow rifle"
               Pmax    Pmuz    burn%   MV
30" bbl         52507   9513    100     3118
25" bbl         52507   11909   99.96   2984
20" bbl         52507   15529   99.23   2805
15" bbl         52507   21356   96.53   2547
10" bbl         52507   31399   87.98   2127
5" bbl          52507   43656   55.48   1287
3" bbl          42961   35718   25.42   617

Note that the max pressure Pmax does not drop until you reduce the barrel length to under 5".

Pmuz is the pressure when the bullet exits the muzzle.

Note that the only change while reducing barrel length is Pmuz (goes up), burn efficiency (down),
and MV (down).

Then I substituted the fast rifle powder and found a load with the same max pressure (close):

FAST rifle powder
20" bbl         52561   10255   100.0   2544
18" bbl         52561   11627   100.0   2477

For the comparison point 20", note that the Pmuz is much lower (about 30%) and the MV is about 300fps
slower than the 20" with the slower powder.

In summary, as long as the barrel is reasonably long -- specifically,
long enough that the max pressure for max load for an indefinitely
long barrel is still reached -- then the slower-powder max load will
have more velocity in the shorter barrel than any other max pressure
load with a faster powder.

Relevant to this discussion is the observation that when the barrel length gets below the bullet distance at max pressure (in a longer barrel), the peak in-the-barrel pressure for a given load is decreased because by the time it would be peaking, it's already left the short barrel. Look at the 5" - 3" bbl data above.

[Matthew_Mink]

mcoliver, are you asking about pistol or rifle loads?

[BSeevers]

Interesting program and since many Supers have been measured at 40,000+ cups or more then there might be a relationship. I am going by years of experience and scientific testing ie: Let me shoot my ammo in your gun and you shoot mine in yours at the range. They mostly don't chrono the same and that's same barrel length and holes, usually. This shorty craze is new so no "man on the street data" from me.

I have seen many accidents and problems with bad ammo created by intelligent people and gave a caution for that reason.

But I don't think a 165 PF load in a 5 will be unsafe in a 4 inch. Heck I and everybody else shot 175+ for over 10 years. Now load a 195+ PF and it will get your attention. You can feel the sting and watch me run behind the car. On a side note I watched a man shoot 190+ .45 in a match. Yes we stopped him after looking at the brass. It was scary, 150 or was it 152? grains too!! Those Penn lead bullets.

[uscbigdawg]

I don't think so, pressure should be the same. The higher velocity is a result of the longer barrel, not higher pressure.

What Matthew said. A 5" load will be 'safe' in a 4" gun and vice versa. It's pressure not velocity is what you're concerned of.

Rich

[old shooter]

My 2 cents.....The answer is maybe/maybe not. Just because a load is safe in one bbl doesn't guarantee that it will be safe in another, reguardless of bbl length or going shorter to longer or longer to shorter. I think that if your load is anywhere max you should work it up just as you would a new load for the shorter bbl.

Mike

[BSeevers]

I don't think so, pressure should be the same. The higher velocity is a result of the longer barrel, not higher pressure.

What Matthew said. A 5" load will be 'safe' in a 4" gun and vice versa. It's pressure not velocity is what you're concerned of.

Rich

So only barrel length effects pressure and velocity?

I suggest that since there are dissenting opinions, mcoliver call Sierra or Hornady and pose the question to them to be safe.

[uscbigdawg]

I think what Matt and I are saying is that barrel length has nothing to do with velocity. Rather it's the load itself and the barrel's chamber.

Bill's advice though is sound. When in doubt, ask the pros.

Rich

[Singlestack]

I think what Matt and I are saying is that barrel length has nothing to do with velocity.  Rather it's the load itself and the barrel's chamber.

Bill's advice though is sound.  When in doubt, ask the pros.

Rich

????

IMO, barrel length has a lot to do with velocity. Just chrono the same load out of a 3in barrel and and then chrono it out of a 5in barrel. Big difference.

[uscbigdawg]

SS,

Whoops had that backwards. Has nothing to do with pressure. It has everything to do with velocity.

BTW, if you're at the GA State, can I see your Mongeese? Look for the tall, slow guy shooting a Viper. Haven't decided if I'm going to where my VP, non-sniper hat backwards, yet. I'll be shooting with TT.

Rich

[benos]

Maybe a good way to think about it is - never assume a load that's near max pressure in one gun will be safe in another. Always back off the load before trying it in a new gun... say 10%.

And yes, barrel length has everything to do with velocity.

be

[Gun Geek]

Yep length has every thing to do with velocity, up to a point.

That's why pistol powders are faster than rifle powders - they must develop pressure quicker and maintain it for a shorter time since the barrel is shorter. OK hang on to your calculus hat - the area under the pressure vs time curve is what really describes a load's ability to do the work (accelerate the bullet).

There's a product out on the market (Pressure Trace) that allows you to measure and plot the pressure vs time curve so you can chose the most efficient/effective load in you gun. You pretty much ignore published loads and start working up until you get the right shaped curve. That's the best you can do in your gun with that powder & bullet.

Just as Zak posted, there is a point where, depending on the powder, velocity doesn't increase (and may begin to drop off). Remember that a certian amount of pressure has to be maintained or the friction of the bullet with the bore will slow down the bullet.

ALL THINGS BEING EQUAL (that's one of those things engineers like to say, knowing full well all things are never equal) Peak pressure in a short barrel and pressure in a long barrel will be the same. The part that has to be equal is the chamber, bore, rifling, etc.

Peak pressure is what destroys things. So, as Brian says, when moving gun to gun or changing a component, back off and work back up. We've heard that advice before haven't we

[mcoliver]

mcoliver, are you asking about pistol or rifle loads?

Matt, Pistol loads.

Good info guys. There's been a rush of really short barrelled guns here (for Modified div.) I was just exploring the possibility of having only one load for my regular Standard gun and a modified gun.

Thanks a lot.

[kevin c]

Yep length has every thing to do with velocity...

So, the longer the barrel, the faster you go?

Thanks! I knew there was a way of explaining it to the wife when she complains...

(sorry, just had to beat Rhino to it... )