Disparity in actual FPS and Powder Charts

[JimGun]

Hodgdon's powders provides a starting load for a .45 ACP 230 gr LRN as 4.5 grs Universal with a FPS of 703. This is with an O.A.L. of 1.200"
M.D. Smith provides a starting load of Universal at 4.5 grs with an expected FPS of 703 for the same bullet, but the website does not provide an O.A.L.
I have seen where some have said they used an O.A.L. of 1.260 - 1.265 with good results for the same bullet.
I loaded a starting load of 4.5 grs of Universal under a MBC .45 ACP 230 gr LRN and a O.A.L. of 1.260, which produced a FPS of 608 - nearly 100 FPS less than Hodgdon's. My question is, could that disparity in FPS be caused by the greater Overall Length?

 

[rfwobbly]

Could be the OAL, could be a lot of things. Each gun is going to shoot the same ammo differently. That's why 1) every reloading manual tells you to work up your own load, and 2) loads from "friends" on the internet are such a bad idea.

Additionally, you will hardly ever get the same results they got while developing the load in the laboratory because their laboratory "handgun" looks like this.....

 

[gamestalker]

There are multiple variables in play that will determine the performance in fps. Brass head stamp, primer brand, LP primer or SP primer type brass, and yes, seating depth is deffinitely a major determining element. There are other less impacting elements also, such as the lot of powder and age of it. And don't forget each individual firearm performs different, even two having been manufactured one after the other, will be individual in chamber & barrel dimensions. And in my opinion one of the most obvious difference in pressure and velocity estimates, is the the test barrel that bullet and powder manufacturer's use to build their projections. I also come to recognize that most of the published loads are over stated, some times by a good deal.
Something I realized early on some 30 yrs. ago is that no matter how precise I am with my reloading process velocities are always going to have pretty good spread. I have found that the higher velcoity, the smaller the percentage of variation in most circumstances. 5% variation with an estimated 700 fps load is only 35 fps in relative terms. But if I had to deal with a 5% variation with an estimated 3400 fps load, I would be looking at a relative variance in fps of around 170 fps, that's significant enough to have a noticable impact on trajectory. Fortunately the variance for higher velocity loads isn't any where near 5%, at least not in my experience any way. I experience something like a 1% or less variance.

 

[cfullgraf]

Here is a single example of different velocities from the same load fired in different, but same style rifles.

I have a Colt HBAR Match Target AR-15 whose velocities with the Sierra 69 grain Matchking gives similar, but not exactly identical, to the velocities listed in the Sierra reloading manual. Interestingly, Sierra uses a Colt Match Target for their test rifle.

I also have an AR-15 Service Rifle match rifle. Externally, it is essentially the same as the Colt. Internally, the match rifle has all the improvements allowed for match work. With the same loads as above, the velocities are about 200 fps faster, about 10%!

While I never questioned the variance in my velocities from the publish data, the reasons offered by all the gun scribes made sense, this example really confirmed for me what was going on.