short barreled rifle question - barrel too short for powder to burn ?

gorky

gorky

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I have a Ruger Compact M77 (16.5" barrel) in 7mm-08. I was messing with it at the range after mounting a new scope and found that I had approximately 5" of drop from 100yds to 200yds. This was using 139g Hornady SST Light Magnum ammunition. This seems like alot of drop (especially when considering that Hornady's website says that 1.5" of drop is expected) so I am wondering if it is caused by:
a) barrel is too short to allow powder to burn completely, and/or
b) the Light Magnum ammunition has a slow burning powder that exacerbates any issue from the barrel being to short.
I am going to try some different ammunition to see what to make of it but I am curious if others have experienced something similar with either this cartridge or with shorter barrels. I rechecked the scope and mounts and all looked good there for those wondering (Bushnell Elite 6500))
 
would heavier bullets help? a little slower accelleration might allow a little more powder to burn off, but I wont make that a promise... that simply speculation...
 
If your rifle is zeroed for 100 yards, the 139 gr SST at 2700 would print 4" low at 200. If you zero the rifle at 200, the bullet will be 2" high at 100, 0 at 200, 5" low at 270 and 8" low at 300.

A chronograph would give you a better idea of what your velocity is, but if your drop is indeed 5" at 200 with a 100 yard zero, that would indicate a muzzle velocity of about 2500 fps.

Shooting heavy bullet loads from a short barrel differ less from long barrel velocities than shooting light bullet loads. This is because the heavy bullet has a lower muzzle velocity and uses less powder. Having said that, the load that produces the highest velocity in a long barrel will also produce the highest velocity in a short barrel. IMHO, you should just sight the rifle in such a way that you can hit out to 300 yards without creating any short range issues.
 
The short barrel on the Ruger Compact will result in very low muzzle velocities.
Most factory velocities are quoted from a 24" barrel.

2500fps from that stubby pipe is not that shabby.
All things considered.
 
With modern smokeless propellent, all the powder is burned in the first few inches of the barrel.The Ruger certainly has enough barrel length to do this!:)

Just sight in as suggested by Boomer.
 
Gatehouse said:
With modern smokeless propellent, all the powder is burned in the first few inches of the barrel.The Ruger certainly has enough barrel length to do this!:)

Just sight in as suggested by Boomer.
Click to expand...

Well not really, Gatehouse. It depends on the powders burn rate, bore, case, etc. Try running 7828 out of a short barreled, straight walled cartridge, and see what kind of fireball you can make!

The rifle the OP has would likely perform better with faster powder then that provided by the "Light Magnum" ammo he is using, such as 4064, Varget or RL15. But if he does not reload, that may not be an option. I do not have a reloading manual in front of me, so some of those powders may or may not be appropriate.
 
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Pudelpointer said:
Well not really, Gatehouse. It depends on the powders burn rate, bore, case, etc. Try running 7828 out of a short barreled, straight walled cartridge, and see what kind of fireball you can make!
Click to expand...

You would have to use a completely impractical powder to not get it to completely burn within the first few inches of the barrel. Using 7828 in a straight walled cartridge might qualify. In this example-the 7-08- all the powder is consumed within 6 inches or so.

The rifle the OP has would likely perform better with faster powder then that provided by the "Light Magnum" ammo he is using, such as 4064, Varget or RL15.
Click to expand...

As Boomer said, the load that produces the highest velocity in a longer barrel will also do so in the shorter barrel.The "fast powders for short barrels" idea is mostly a myth, unless we again are using completely impractical powders for the task at hand. Varget, RL15, etc will work well in a 7-08 regardless of barrel length.
 
Gatehouse said:
]

You would have to use a completely impractical powder to not get it to completely burn within the first few inches of the barrel. Using 7828 in a straight walled cartridge might qualify. In this example-the 7-08- all the powder is consumed within 6 inches or so.



As Boomer said, the load that produces the highest velocity in a longer barrel will also do so in the shorter barrel.The "fast powders for short barrels" idea is mostly a myth, unless we again are using completely impractical powders for the task at hand. Varget, RL15, etc will work well in a 7-08 regardless of barrel length.
Click to expand...

Huh? :confused::confused::confused::confused:
I think this is where a lot of misinformation might cause some confusion.

Mathematically it is the area under the pressure curve that shows the velocity or the potential for velocity. Changing burning rate changes the shape of the pressure curve. If we go back to high school calculus we can figure the area out. Unfortunately the only way for the laymen to get this info was with a PB43 and a strain gauge. The pressure curve is drawn along the length of the barrel and once the bullet leaves, the curve stops so a shorter barrel has a shorter length to measure the area.

So, we can change the shape of the curve with the different burning rates and optimize for different barrel lengths.

As far as the first few inches of the barrel......who has seen the flash at the end of a barrel in low light conditions?
 
The flash from the barrel you see at low light is most likely hot gases, not unburned powder being burned outside of the barrel.
 
I have the same rifle and what I did was used the pistol data for 7-08 and worked it up watching for pressure. The faster powders seem to work well but I have to load some of the slower ones yet and chrono them.
 
Mathematically it is the area under the pressure curve that shows the velocity or the potential for velocity. Changing burning rate changes the shape of the pressure curve. If we go back to high school calculus we can figure the area out. Unfortunately the only way for the laymen to get this info was with a PB43 and a strain gauge. The pressure curve is drawn along the length of the barrel and once the bullet leaves, the curve stops so a shorter barrel has a shorter length to measure the area.

So, we can change the shape of the curve with the different burning rates and optimize for different barrel lengths.

It's probably simpler to just read up on actual experiments with different powders and shortening barrel length and how the highest velocity powders in long barrels were also the fastest when the barrel was shortened.




Joep17 said:
As far as the first few inches of the barrel......who has seen the flash at the end of a barrel in low light conditions?
Click to expand...

You mean the hot gasses hitting the air and igniting?:p

it's not "unburnt" powder.
 
Gatehouse is correct. The powder has deflagrated within the first few inches of barrel, and is converted into hot, expanding gases. These gases are what push the bullet down the bore, and when you shorten a barrel, the gases are released into the atmosphere sooner than they would be from a longer barrel, effectively decreasing the energy that they impart to the bullet. The flash you see at night is from the gases igniting (oxidizing) as soon as they hit an unlimited supply of oxygen in the atmosphere.

We have to remember that pressure is the main factor in the burning of smokeless powder. Low-pressure cartridges/loads may not completely burn a slow-burning powder in a short barrel.
 
I am keeping my eye on this thread as I am interested in a short, compact rifle. I was looking at my Speer reloading manual last night and a pistol in 7mm-08 with a 14" barrel was down a good 400fps, iirc. With a 20" barrel I was ~2700 fps with 140 grain bullets out of my Model 7 SS in 7mm-08 and just over 2400fps with 160gr bullets.

Here is a question that someone with experience in the matter may be able to answer. If you take a Tikka Battue, they have just under 20" barrel, and grab one in .300 Win Mag and one in .300 WSM, how will they compare?

Specifically, look at this data out of a 24" barrel:

Both using IMR 4350 and 150gr bullets

.300 WSM - 68.0gr - 3221 fps
.300 Win Mag - 74.0gr - 3254 fps

Varget and 150gr bullets

.300 WSM - 58.5gr - 3140 fps
.300 Win Mag - 60.0gr - 3108 fps

What are your predictions for the loss of 4" of barrel?
 
Some interesting data on the IMR/Hodgdon site.

7mm-08; 15" barrel pistol data vs 24" barrel rifle data using 139 gr Hornady Spire Point.

Max loads are the same, as are max pressures.

H4350 - 50.0gr charge (slowest powder given)
15" - 2581 fps
24" - 2906 fps (fastest in the rifle)
Difference - 325 fps

Varget - 43.5 gr
15" - 2612 fps (fastest in pistol)
24" - 2857 fps
Difference - 245 fps

H4895 42.5 gr
15" - 2575 fps
24" - 2857 fps
Difference - 282 fps

So it does appear that the faster powders gain on the slower powders in a shorter barrel, and the faster powders also lose less velocity. In this case anyways.
 
1899 said:
So it does appear that the faster powders gain on the slower powders in a shorter barrel, and the faster powders also lose less velocity. In this case anyways.
Click to expand...

What it actually shows is that the pistol they used got higher velocity with Varget and the rifle got higher velocity with the H4350. They didn't use the same firearm and shorten the barrel when they did their load testing.

The only way to do an apples to apples comparison is to shoot the rifle (24" barrel) and then chop it down to 15" and shoot again.

Looking at the data you posted, all the 15" and 24" velocities are within 50fps of each other, and it could probably go some other way with different firearms.
 
1899 said:
I am keeping my eye on this thread as I am interested in a short, compact rifle. I was looking at my Speer reloading manual last night and a pistol in 7mm-08 with a 14" barrel was down a good 400fps, iirc. With a 20" barrel I was ~2700 fps with 140 grain bullets out of my Model 7 SS in 7mm-08 and just over 2400fps with 160gr bullets.

Here is a question that someone with experience in the matter may be able to answer. If you take a Tikka Battue, they have just under 20" barrel, and grab one in .300 Win Mag and one in .300 WSM, how will they compare?

Specifically, look at this data out of a 24" barrel:

Both using IMR 4350 and 150gr bullets

.300 WSM - 68.0gr - 3221 fps
.300 Win Mag - 74.0gr - 3254 fps

Varget and 150gr bullets

.300 WSM - 58.5gr - 3140 fps
.300 Win Mag - 60.0gr - 3108 fps

What are your predictions for the loss of 4" of barrel?
Click to expand...

I predict you will lose about 150fps in both. Maybe a bit less, but probably not much more..But it's just a guess as I've never shot a 20" WSM, although I do have a 20" 300RUM sitting in my shop right now.:D
 
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