Hi, just a quick question I hope. I was wondering if anyone out there has some info on the correlation between barrel length and how it effects muzzle velocity? The test gun in the Hornady manual I have are 22 inch barrels and my rifle has a 22 inch barrel. The load in question is a 30-06 with a Hornady 180 gn spitzer with 48 gns of IMR 4064. The manual's chart says that 48.7 will yeild a muzzle velocity of about 2700 fps and my "Chrony" says the muzzle velocity is about 2500 fps. The test rifle and mine are both M70 Winchesters. Now the Sierra book says they used a 26 inch barrel to achieve the same muzzle velocity with the same powder. I know my load is .7 grains short of the published dose of powder but I cannot understand the big difference. If I follow the Hornady manual I should be achieving about 2650 fps. "Is it the CHRONY"?

Your mistake is that you are expecting that a given load will produce a given velocity in all guns.This does not happen even if all of the guns have the same barrel length.Variations in barrel and chamber dimensions as well as differences in various lot#s reloading components can cause velocities to vary considerably.Although a load will usually produce a higher velocity in a longer barrel,it is very possible to have a given load produce more velocity in a 22" barrel than it does in a different 24" barrel.I don't suppose that you are also expecting the load that was the most accurate in the gun used in the manual to be the most accurate in your rifle as well?Use manuals as a guide but don't take them too literal or you will be disappointed on a regular basis.

Test barrels used to develop loads are cut to minimum SAAMI dimensions. This creates pressures and muzzle velocities that are higher than will be found in an average rifle. The actual difference in MV varies considerably, but 70 fps difference between published and actual is a ballpark number.

MV as a function of barrel length depends on several things, but for a 22" barrel, it's around 30 fps per inch. When you get to a long barrel, like my milsurp Swede, 29", it's more like 15-20 fps per inch.

Hodgdon uses a selected lot of powder for testing. The powder is right in the center of the spec for that particular type of powder. Your individual bottle of powder might produce higher or lower pressures and MV's.

Stubblejumper, I'm not disappointed and I do expect there to be varience, I just didn't expect it to be that much. In the load development I did try different powders and weights and this particular combination was the best shooter (a little more that a 1/2 inch @ 100 yds) which is plenty good for my hunting application. I do not think I have made a mistake in inteperting that, the data published will be diffrent from actual results. I did not think it would be exact but I did believe it would be somewhat closer than my results. I try not to make mistakes and if I do I learn from them. I am not an idiot and wish you wouldn't percieve me as such. An informational answer to the post with a kind attitude trying to help someone find the answer to a question would have been more appreciated. What is it that you gain from trying to belittle people asking a question?

There's also the question of how can you be sure your Chrony's accurately calibrated?

PETEM-I did not imply that you were an idiot and my intent was not to belittle you.My point is that most people with limited experience in reloading mistakenly assume that the data in reloading manuals can be taken literally.They assume that it is all black and white and that a given load will produce a given velocity with a given barrel length.Many still assume that any load listed in a manual will be safe in their gun.They assume that if a load is accurate in one gun it should be accurate in others.Generally it takes some experience for most people to realize that reloading manuals should only be used as a guide as the velocities,pressures and accuracy that are listed in reloading manuals can vary greatly from the results obtained in your own guns.