Barrel length vs. powder burn

[frostop]

Is there a direct correlation to how well a given powder will work in a given length barrel?

The reason I ask is I have been working on a load for a 270wsm 26" barrel, initially i have been using Retumbo, I have been getting good groups at 100yds but lousy SD's. Then yesterday I loaded up some H1000 and I really didn't even pay attention to the group size but got a single digit sd?

So I have to wonder if Retumbo would really like a longer barrel to burn the powder to a more stabilized point?


Gary

 

[ar10ar15man]

answer is not that simple

 

[frostop]

Yeah, I realize that. I guess I am just wondering if anyone else has seen similar results? A lot of guys run long barrels on this forum and other run light weight mountain rifles and I would be interested in weather they are using a powder like Retumbo or 4350 and what barrel length/bullet combination etc?? Might save some guys money, time and barrel life.

 

[g0rd0]

Case capacidy, internal barrel length, and flat base or boat tail are only the tip of the ice burg. Unless you have a Prowley slide rule or are a balistic savont just go with start load on confermed published data and work up. You may get lucky and find something quickly. Over the years I have had to start from scratch many times, due to components been discontinued or just unavalible in the market at the time. When you do find a combo that you just love run out and buy as much of those components that you can and hoard!
but, really dont get caught up in the my load is faster than your load crap. 3150fps with a 150grn pill dont mean **** if you cant hit the side of a barn.

 

[BallisticsGuy]

The smaller the bore, the heavier the bullet and the more the case volume slower the powder needs to be to not reach catastrophic pressures and thus the longer the bore needs to be for a complete burn (for the purposes of this discussion, let's call a full burn "that barrel length where you cease seeing any muzzle flash while shooting in the dark.")

Here's a set of examples that show what's going on a little bit. I've normalized them for bullets on the light side of the range you're likely to find normally in each.

.22LR (36gr) tiny case capacity, .22" bore, very short pistol barrel required for a full burn and a very fast burning pistol powder being used.
.224BOZ (55gr), small case capacity, .22" bore, moderately long pistol barrel required for full burn and a slow burning magnum pistol powder being used.
.223rem (55gr) moderate case capacity, .22" bore, carbine length rifle barrel required for full burn and a relatively fast burning rifle powder
.223WSSM (55gr) large case capacity, .22" bore, full length rifle barrel required for full burn and a relatively slow burning rifle powder.

As the case grows or the bore diameter shrinks or the bullet gains mass you have to slow down the burn speed and increase barrel length to get a full burn.

For the pedants: Yes, I've trivialized a lot but it should at least get the point across.

 

[entoptics]

In my experience, when you're in the range of "normal" barrel lengths, there's probably no measurable correlation to barrel length between powders with burn rates as similar as H1000 and Retumbo. The cartridge and bullet combo has 99.9% of the leverage regarding powder performance in terms of ES/SD.

In other words, a good load with low ES/SD in a 24" barrel will also have good ES/SD in a 30" barrel, all things being equal. Most of the "action" happens in the first foot or so, if not the first inch (e.g. neck tension, throat, concentricity, powder column, etc).

Obviously, if you're talking drastic changes in barrel length (e.g. 14" vs 28"), some of the above isn't applicable.

 

[therifleman556]

Retumbo is almost a touch too slow even with 150 grain bullets. I ran out of case and didn't see pressure with the 145 ELD-X. Seating the bullets .010" off of the lands, 71.5gr in a Winchester case is heavily compressed and ran right at 3200 in my 24" barrel. I plan on reaming the throat ~.080" deeper and run it as a long action to squeeze a little more in there.

The 150 Berger (in my rifle) is almost a perfect fit, touching the lands it fits in the magazine and doesn't hang onto the case like the Hornady does.

Magpro has long been the go to powder for speed in the 270 wsm but some say it's finicky with temp swings. I plan to try 7977 which is close in burn rate but supposedly temp stable, and 7828 which is close to your H1000.

 

[Canhunter35]

It's not that simple.
IMO Build loads with both powders and compare which is best.
Ime, don't worry about sd and es of your load until you start seeing vertical stringing on your target, it just doesn't matter until then

 

[Susquatch]

Yeah, I realize that. I guess I am just wondering if anyone else has seen similar results? A lot of guys run long barrels on this forum and other run light weight mountain rifles and I would be interested in weather they are using a powder like Retumbo or 4350 and what barrel length/bullet combination etc?? Might save some guys money, time and barrel life.

To be frank, I don't think full burn is a worthy goal - or at least not one worth worrying about. As has been said, start with book recommendations and find what works for you in terms of velocity and ES.

I'm guessing that max velocity (with acceptable pressure) will probably coincide with lots of unburned powder because pressure to drive the bullet starts to fall off after the powder quits burning. Also, powder burn completion isn't an instantaneous event - rather, the burn rate slows down and then eventually stops.

From an engineering perspective, max velocity would result from constant max pressure and constant acceleration from start to finish. That isn't really possible with the powders we have today, but does explain why big cases result in higher velocities within acceptable pressure limits.

Personally, I like to look at case fill and velocity to choose the best powder to work on. I believe (but have no proof) that case fill is a better starting point when going after the highest velocity, lowest ES, and best accuracy.

Usually, case fill goes up with slower powders and down with faster ones. Look at the charts to see which powder gets closest to 100% at max pressure, then start lower and work up. This will usually also be the highest velocity other than compressed loads (which I avoid).

I don't even think about burn completion.

That all said, your observation of low ES might be interesting to think about from a purely technical perspective. I just wouldn't let something like that get in the way of developing a great load.

Just my two cents about a complicated issue.

 

[frostop]

Well, this is where I'm at! When I built this rifle, I researched this site for powder options and Retumbo seemed to be mentioned a fair amount so I bought Two pounds.
I have tried seating out to just shy of the lands and back down to just inside the magazine length and really never saw SD's that I liked. So the other day I got down to the last bit of the Retumbo and decided to load a minimum load of the H1000 seated just inside the mag, which shot in the 2900 ft range with an sd of 9, five shots! This is a hunting rifle so I would like to keep it inside the mag and I would like to stretch it out to say 700 so I would like to keep the SD's down in the single digits.

Unfortunately, I have gone in search of powder, H1000, locally and haven't found any and probably going to have to order it! Planning on trying it out!

Thank you for all the reply's, always a wealth of knowledge and experience here, I will just continue as usual and hopefully H1000 will be the one for this rifle

Gary

 

[ptsdvet]

I felt this was good discussion BUT let's say in my 6.5x 284 a load reaches 2950 and shoots a constant 3/8 group yet case capacity is only 85% and could stuff in 8 more grains be just off the lands slightly compressed load, no pressure or primer sings reaching 3250 But the groups is 3/4" The Question is speed or accuracy?

 

[Susquatch]

I felt this was good discussion BUT let's say in my 6.5x 284 a load reaches 2950 and shoots a constant 3/8 group yet case capacity is only 85% and could stuff in 8 more grains be just off the lands slightly compressed load, no pressure or primer sings reaching 3250 But the groups is 3/4" The Question is speed or accuracy?

Excellent point and good to have you remind everyone when we get too focussed on one parameter.

In my mind, Accuracy trumps all other results EXCEPT when ES is so bad that 600+ groups go to hell in a handbasket. When that happens, you have to compromise.

That's why long range competitors are so anal about ES vs the short range Bench Rest folks who don't pay it much attention.

 

[Canhunter35]

If 700 is your parameter, take both loads and test at that range. Then choose the best one. Let the targets tell you your best load.
Good sd does not always equal accuracy

 

[Susquatch]

If 700 is your parameter, take both loads and test at that range. Then choose the best one. Let the targets tell you your best load.
Good sd does not always equal accuracy

I agree with CanHunter 100%. I'd only add that the comparison should include wind as well because velocity affects wind drift too.

I'm just not sure why he likes cans over game....... LMAO!