Velocity in a 450BM bolt gun from a 16 vs a 20" bbl

I know of one so far, let's see if more show up.

Moderator: MudBug

Velocity in a 450BM bolt gun from a 16 vs a 20" bbl

Postby Trapaddict » Sun Nov 22, 2015 8:42 pm

New to the site tonight and I have yet to figure out the search feature on this site. :?: I am sure this has been discussed at great length somewhere on the site but I cannot locate it. I am in the process in putting together a southern Michigan whitetail rifle for next year as well as one to be be used in close quarters blinds in heavy cover in Northern MI. I settled on a 17.5" barrel with a varmint contour on a Savage single shot action with the factory pillar bedded stock. I have several of these guns in different calibers and they all shoot like a house afire if I do my part. Does anyone here have first hand experience with the same load out of two length barrels and was there any appreciable velocity drop? I have read conflicting information about the 450BM with regard to powder burn. Some state that there is compete burn in a 16" barrel while others claim almost a 10% increase when stepping up to a 20" tube. Who is right?

Jeff
Trapaddict
 
Posts: 2
Joined: Sun Nov 22, 2015 6:58 pm

Re: Velocity in a 450BM bolt gun from a 16 vs a 20" bbl

Postby Hoot » Mon Nov 23, 2015 8:36 am

Welcome aboard Jeff. The point where all the powder is burned is not always the point where additional velocity yield occurs, though it's quite often the point of diminishing return. As you would imagine, the burn characteristic of the powder factors in. Faster burning powers peak sooner as the bullet moves down the bore. See my signature as it applies here. According to QuickLoad modeling software, the difference in velocity with a load pushing the Hornady 250 FTX ahead of 38gr of Lil Gun, is at 16 inches, the bullet is moving 2171 fps and at 20 inches, 2238 fps. Net increase being 67 fps. While the velocity is akin to Hornady Factory loads in velocity, I suspect they use a slightly slower burning powder, so a reported 100 fps may indeed be accurate. Reported velocities are anecdotal in that they rely upon one persons rifle's makeup. Minor differences in bore finish, bore diameter, bore condition, etc, can make a measurable difference in velocity, not just the length. I'm not going to get into temperature when the testing is done because again, we all know that ball powders burn faster as their temperature increases. It just means that testing different barrel lengths, all else being equal, must be done under the same conditions. IE not cool morning vs warmer afternoon on the same day.

Here's an image from QuickLoad showing graphically the bullet velocity as in theory it races down the bore.

Image

Hoot
In Theory, there is no difference between theory and practice. In Practice, there is.
User avatar
Hoot
 
Posts: 5083
Joined: Tue Feb 09, 2010 9:34 am
Location: Minnesota

Re: Velocity in a 450BM bolt gun from a 16 vs a 20" bbl

Postby Al in Mi » Mon Nov 23, 2015 10:05 am

Jeff, if you want a longer barrel, get with Jim at Apache Gunworks. He did mine in 24" for a 700 but sure a Savage is no problem.

I have some chrono data at work, but be next week before I get back there.
User avatar
Al in Mi
 
Posts: 1982
Joined: Sat Jun 20, 2009 8:24 am

Re: Velocity in a 450BM bolt gun from a 16 vs a 20" bbl

Postby pitted bore » Mon Nov 23, 2015 12:24 pm

Trapaddict wrote: New to the site tonight and I have yet to figure out the search feature on this site. ... I have read conflicting information about the 450BM with regard to powder burn. Some state that there is compete burn in a 16" barrel while others claim almost a 10% increase when stepping up to a 20" tube. Who is right? ...

Jeff-
Welcome to the forum! It's nice to have more bolt-gunners participating, and more Michigan persons, even if they do live south of The Bridge.

The search feature built into the forum software is pretty pathetic. It's better to use something like google and specify the site to be searched along with the search words.

The notion of an increased powder burn with longer barrels is a misconception. All of the powder that is going to burn will burn in the first very few inches of the barrel. Hoot's graph shows the effects of this, with pressure peaking after about an inch of barrel travel. A longer barrel will increase velocity because there is more time for the barrel pressure to accelerate the bullet. (The pressure has to remain sufficiently high to overcome bullet-barrel friction. In 22 LR, barrel lengths more than about 18 inches produce lower velocities because pressure has dropped below the level that can accelerate the bullet.)

This is why it is almost always observed in comparisons of different loads that the powder-bullet combinations producing the highest velocity in a short barrel will produce the highest velocity in a longer barrel.

Please keep the forum updated on your bolt-action experiences.
--Bob
User avatar
pitted bore
 
Posts: 700
Joined: Sun Jun 07, 2009 8:05 pm
Location: The U.P.'s U.P.

Re: Velocity in a 450BM bolt gun from a 16 vs a 20" bbl

Postby IceAxe » Wed Nov 25, 2015 1:20 pm

PropBurnt.gif
PropBurnt.gif (119.93 KiB) Viewed 21063 times
I believe the graph that Hoot posted shows peak pressure not propellant burned. Here is a printout that shows an idea of percent of a powders burn.
NRA Life
User avatar
IceAxe
 
Posts: 68
Joined: Fri Aug 15, 2014 9:43 pm
Location: Colorado Springs, CO

Re: Velocity in a 450BM bolt gun from a 16 vs a 20" bbl

Postby pitted bore » Wed Nov 25, 2015 11:43 pm

IceAxe wrote:I believe the graph that Hoot posted shows peak pressure not propellant burned. Here is a printout that shows an idea of percent of a powders burn.

IceAxe-
Thanks for posting the QuickLoad printout. I'll assume you were responding to my post immediately preceding yours.

I tried to qualify my statement about powder burning by stating that the pressure curve shows the effects of the powder burning. I did not mean to imply that all the powder had been consumed when pressure peaked.

I did err in not similarly qualifying my statement: "All of the powder that is going to burn will burn in the first very few inches of the barrel." I should have written "Almost all of the powder..."

Here's a QuickLoad graph that was posted on the Firing Line forum on a 2013 thread: Powder Burn in Chamber or Bore?
.
Image
.
I have edited the graph for smaller width, and added the Blue Arrows. For this cartridge with the stated load, the left-hand blue arrow shows that 80 percent of the powder is burnt in the first four inches of the barrel, and the right-hand arrow shows 90 percent burnt in the first seven inches. About 60 percent of the powder is burnt at the point of max pressure, when the bullet has moved about two inches. Hoot's graph shows the peak pressure at one inch for the Bushmaster, with the differences due to the powders and cartridges.

It's likely that the burning of the last 10% of the powder contributes negligibly to an increase in pressure and velocity, because the volume of the chamber and bore have increased substantially. So, the burning of the final 10% of the powder is going to raise the pressure behind the bullet comparatively little, with relatively small resulting increases in velocity.

All of the above is from my reading and correspondence, since beyond inpection of fired cases for gross signs of pressure, I've collected no pressure data myself. However, it does pretty much match what I've seen.

Thanks again.
--Bob
User avatar
pitted bore
 
Posts: 700
Joined: Sun Jun 07, 2009 8:05 pm
Location: The U.P.'s U.P.

Re: Velocity in a 450BM bolt gun from a 16 vs a 20" bbl

Postby Hoot » Thu Nov 26, 2015 12:02 am

FWIW, in the graph I posted earlier, though its hard to see, there's a dashed vertical magenta line that shows where the powder has theoretically finished burning.
That having been pointed out, what Bob says is spot on. The high pressure initially is due to the propellant releasing a large amount of gas into a small area. As the bullet races down the bore, that area increases at a much greater rate than the remaining propellant releases gas to fill it. As he pointed out as well, even though at some point, there is no additional propellant left to burn and release gas, the bullet will still take advantage of a longer barrel. That pressure, though dropping, is still pushing that bullet until it breaks the seal as it exits the muzzle. It is though, a diminishing return if the powder peaks too soon. If one chooses the proper powder that burns at a rate where it takes longer to release all of its gas, nearer to the muzzle on a longer barrel, it is reasonable to expect a greater gain in resultant velocity than with a powder that exhausted all its gas further back. Those slower burning powders typically has more energy to release than the quicker burning powder as long as the bullet has not broke the seal before it is done doing so.
If I haven't done a good enough job obfuscating this principle, I can always go on a little more. ;)

Hoot
In Theory, there is no difference between theory and practice. In Practice, there is.
User avatar
Hoot
 
Posts: 5083
Joined: Tue Feb 09, 2010 9:34 am
Location: Minnesota

Re: Velocity in a 450BM bolt gun from a 16 vs a 20" bbl

Postby IceAxe » Thu Nov 26, 2015 10:04 am

I totally, agree with all of the above and this burn characteristic is probably more exemplified in straight walled cases. With regard to barrel length, as long as there is an unbalanced force there will be acceleration and considering that Force is equal to mass multiplied by the change of velocity over time. The magnitude (pressure) of the impulse has diminished considerably and continues to drop beyond 16 inches, coupled with the velocity the projectile is traveling at while in the last 4 inches (16-20) won't add much proportionally to the duration of that impulse or the resulting velocity, but it will add some. Personally, I am really enjoying my 20 inch barrel. However my rational for getting a 20 inch barrel was more a gas system related choice. I wanted to go with a mid length gas port to help prevent early unlocking with warmer loads yet have enough barrel remaining to provide a sufficient pulse width to cycle reliably. I also chose to go with a larger bolt and extension.
NRA Life
User avatar
IceAxe
 
Posts: 68
Joined: Fri Aug 15, 2014 9:43 pm
Location: Colorado Springs, CO

Re: Velocity in a 450BM bolt gun from a 16 vs a 20" bbl

Postby Al in Mi » Thu Nov 26, 2015 12:44 pm

IceAxe wrote:I totally, agree with all of the above and this burn characteristic is probably more exemplified in straight walled cases. With regard to barrel length, as long as there is an unbalanced force there will be acceleration and considering that Force is equal to mass multiplied by the change of velocity over time. The magnitude (pressure) of the impulse has diminished considerably and continues to drop beyond 16 inches, coupled with the velocity the projectile is traveling at while in the last 4 inches (16-20) won't add much proportionally to the duration of that impulse or the resulting velocity, but it will add some. Personally, I am really enjoying my 20 inch barrel. However my rational for getting a 20 inch barrel was more a gas system related choice. I wanted to go with a mid length gas port to help prevent early unlocking with warmer loads yet have enough barrel remaining to provide a sufficient pulse width to cycle reliably. I also chose to go with a larger bolt and extension.


who's barrel you end up going with?
User avatar
Al in Mi
 
Posts: 1982
Joined: Sat Jun 20, 2009 8:24 am

Re: Velocity in a 450BM bolt gun from a 16 vs a 20" bbl

Postby IceAxe » Thu Nov 26, 2015 11:04 pm

It's a Blackhole Weaponry, Jay at BHW was kind enough to do the custom work and use the AR Perf. 800 series bolt. I was concerned that it might not feed properly with the larger bolt and possibly hang on the case mouth. As it turned out it feed great and I haven't had any issues with it's operation. As well as it works I wouldn't mind picking up another one but they are never available. At least not on the site when I check.
NRA Life
User avatar
IceAxe
 
Posts: 68
Joined: Fri Aug 15, 2014 9:43 pm
Location: Colorado Springs, CO

Next

Return to Bolt Action Rifles

Who is online

Users browsing this forum: No registered users and 11 guests