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.

My 2 Browning Abolts in 300 win mag shoot a 180 gr sierra at 3150 fps and 2960 fps with the exact same load of RL22 that is 1.5 gr short of max in the Alliant manual! By the way, the gun that shoots 2960 fps is bolt heavy and is thus overloaded at this less than max charge. I recently sold that rifle. ALL barrels are different, even from the same manufacturer.

Are the timers, sky screens, the correct distance apart? Do you have low readings with other guns?

By definition all of our chronographs are "wrong" by some amount. I had one that was off (slow) by at least 75 fps. Not only that but its winter out (here) and things are slugish. There is a small adjustment to actual vrs instrumental velocity also.



I suppose one could piggy back yours with a lab instrument. I have three of them here. I just use the Oehler 35P now.

PETEM, This is a most interesting question, which has generated thoughtful responses. To answer your original question, yes there is a correlation between MV and Bbl. length, but it isn't 1:1 or perfect /i]. This simply means that factors other than bbl. length contribute to velocity; e.g., actual bore diameter, smoothness of bore, bearing length of bullet, hardness of bullet jacket, brand of brass (internal capacity) used, etc. ...Maven

All, Sorry for the italics: Only the phrase, "but it isn't 1:1 or perfect" should have been emphasized. ...Maven

PETEM, just to be a bit more complete, all the following will affect muzzle velocity, and I'm not even sure this is a complete list:

1. Barrel length

2. Tightness of the chamber

3. Length of the freebore

4. Seating depth of the bullet

5. Temperature of the ammunition

6. Temperature of the chamber

7. Amount of powder

8. Characteristics of the specific lot of powder

9. Type of primer, though often not a lot

10. Barrel inside diamter

11. Capacity of the brass case

Modern chronographs are hard to uncalibrate. The electronics are regulated by a crystal controlled oscillator, and that just doesn't vary enough to make any difference that you would care about... roughly 10 ppm per degree C. Lighting, and the position of the sunscreens can make minor differences, up to about 25 fps. Unless you have done something peculiar, if you have a modern chronograph, it's probably every bit as accurate as the ones used to generate published data. That applies to the expensive ones, and the cheap ones as well. The $69 Shooting Chrony is repeatable to about 1 fps.

My circa 1987 Pact started reading low when I bought the new screens and screen holders from them. Up to that point I used my home made screen holders that I measured the spread on and they were five feet as well. It seems that the new screens are not aiming straight up. I reversed one of them per Pacts suggestion and the velocity seemed to come up. When warms up I will set it in tandem with the Oehler and tweak it to agree.

It seems to have been 75 fps slow.

Petem!What Denton is saying cuts directly to what you are experiening. What the rest have said is correct but Denton's blurb really encompasses the causes for the effect that you are witnessing.Barrel length is only another contributer to how fast that bullet comes out. Welcome aboard. Roger