OBT method, barrel length vs velocity

[Stammster]

You have a 20.5 inch barrel. A speed of 3000+ is really good out of such a barrel. Think about driving with your car. You don't keep the revs at maximum when driving as it saves on wear and tear. Running at 65K psi puts extra stress on your lugs and cases. Rather use a longer barrel to get extra pace.

It all depends on your mission. Shooting 1 shell per year at a trophy elk at 600 yards vs match shooting every weekend.

Also short barrels have their place. You obviously haven't hunted hogs in Texas. Bailing out of a moving UTV is a lot easier with a short barreled gun in one hand, especially while holding a Bud Light in the other. As I said above (post 24), this gun is set up for a light load. I only ran the full charge weights during the initial load development.

Back to the main topic - OBT. This gun/load is case in point where the best accuracy did not occur at a recommended OBT node. But in full disclosure it was only one 3 shot group and maybe something else was going on.
Also velocity "node" derived from recommended OBT seems to be independent of barrel length.

 

[Tripodmvr7]

Thanks.
Questions to ponder
1.). So why the bad group so close to the ideal?
2.). 3050 fps +/- 10 fps seems to be the recommended "ideal" OBT based muzzle velocity for this node. My original argument / question was around this being the same for multiple barrel lengths. So why not just simplify it to that. Let people strap on a chronograph and aim for a charge weight that gets them there.

Why not simply just give the following:
Node 4: 3270fps
Node 5: 3060fps
Node 6: 2930 fps
Etc.
Am I trying to oversimplify things? Perhaps, but IMO so are OBT (This is dictated by physics, being the speed of sound in steel and can therefore be calculated if barrel length is given) times...but in reality it's just appears to boil down to the best velocity to target - essentially independent of barrel length. So no need to even involve time - as it seems to cancel out. Your rifle is a unique platform and the case volume (or chamber volume really) is different and affects the burning characteristics of the propellant) Jump to lands and bullet used also affect barrel time. The burning characteristics of the propellant also dictates time spent in barrel. Faster burning powders have a reduced node speed, as they give fast accelaration initially but then lose pace. Slower powders might have a slow start but sustain pressure pressure better and get a faster barrel time. The request to supply measured data of your rifle is to have accurate information that will not give false calculated values. A one grain change in case volume relates to 27fps in your rifle.

A node is also not an accuracy point, but rather a band with a width of up to 1 gr of propellant. That is something that you must find experimentally - more range time.

I'm also going to make some runs with other calibers and bullets weights to see if this crosses over to other calibers. Initially my 25-06 data is showing it's pretty close with node 2 at 3790 fps and node 3 at 3390-3460 fps and node 4 at 3220-3270 fps, with 85-100 gr bullets.

 

[Stammster]

Tripodmvr7
Thanks for all the help. This OBT method will be one more tool in the toolbox to help cut down on my load development time - but not used in isolation.

 

[rammac]

I've had over 10 years of experience using Quickload to develop my handloads and I accept the idea that I'll have to test the ammo and adjust the load accordingly but the key advantage to Quickload is that I use far less of my supplies getting to that optimized load. The latest Quickload development took me exactly 20 rounds to find a consistent sub MOA, 180gr load for my new AR .308, short barreled rifle. I've used that load out to 400 yards and I can hold MOA or better groups (5 rounds at a minimum). Not bad results for an out-of-the-box 16 inch barreled Springfield Saint. And I'm pushing that 180gr bullet up to just under 2400 FPS without excessive pressures.

 

[asd9055]

I've had over 10 years of experience using Quickload to develop my handloads and I accept the idea that I'll have to test the ammo and adjust the load accordingly but the key advantage to Quickload is that I use far less of my supplies getting to that optimized load. The latest Quickload development took me exactly 20 rounds to find a consistent sub MOA, 180gr load for my new AR .308, short barreled rifle. I've used that load out to 400 yards and I can hold MOA or better groups (5 rounds at a minimum). Not bad results for an out-of-the-box 16 inch barreled Springfield Saint. And I'm pushing that 180gr bullet up to just under 2400 FPS without excessive pressures.

Totally agree!

 

[cmysix]

I have read just recently that it's about 25 FPS per inch and they arrived at that by cutting an inch off a barrel at a time

 

[Tripodmvr7]

It all depends on your mission. Shooting 1 shell per year at a trophy elk at 600 yards vs match shooting every weekend.

Also short barrels have their place. You obviously haven't hunted hogs in Texas. Bailing out of a moving UTV is a lot easier with a short barreled gun in one hand, especially while holding a Bud Light in the other. As I said above (post 24), this gun is set up for a light load. I only ran the full charge weights during the initial load development. I mostly hunt in our northern bushveldt with many thorns and dense bush to get through. My favorite rifle is a Ruger No. 1A in 30-06. The short action and 22 inch barrel means very little barrel getting stuck on branches and still plenty oomph to kill with the 150TSX.

Back to the main topic - OBT. This gun/load is case in point where the best accuracy did not occur at a recommended OBT node. But in full disclosure it was only one 3 shot group and maybe something else was going on.
Also velocity "node" derived from recommended OBT seems to be independent of barrel length.