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Muzzle velocity estimate without Chronograph

13224 Views 22 Replies 9 Participants Last post by  BKeith
I don't have access to a chronograph at this time and I don't want to spend the money. And I just need a rough rule of thumb or experience with this matter.

So what should I expect the muzzle velocity difference to be when using a
Rock River AR15 with a 16 inch barrel 1 in 9” rate twist instead of a
Remington 700 26”, 1 in 12” twist (Hornady Data used from their reloading book)?
My guess is a loss of 150 fps.

I reloaded the 223 Rem using Hornady Data:
55 gr #2266 SP w/c
B.C. 0.235 C.O.L.: 2.200”
Varget 25.5 gr
Velocity per book 3100 FPS
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Jason got it right in principle. The only fly in the ointment is that the Hornady case is likely to go into the AR with more like 30 grains water capacity than QuickLOAD's default 28.8, and to eject with more like 30.5 grains water capacity. I base that just on having measured cases fired through my own AR. The result I got is 3092 fps for the long barrel and 2732 fps for the short, so 360 fps difference. I also have a second, slightly faster powder file for Varget based on some other measurements I made, and it adds a couple percent to pressure and about 40 fps. Yours should wind up in that range somewhere.

How much you lose will vary a little with the powder choice. I find Varget is really a little slow burning for best work with the 55 grain bullets, though some get good accuracy from it anyway. In my guns it does best with match bullet weights from 69 grains and up. With 55 grain bullets I have an easier time getting accuracy out of something like Reloader 10X, which, at just 22-22.4 grains (less expensive to use) gives you the same pressure as my two Varget models, respectively, but adds 43 fps to the long barrel and 51 fps to the short barrel velocity. So it produces slightly less velocity difference at 352 fps.

There are other choices that nudge the velocity higher, but waste more powder. If I try to keep the charge at about 90% case fill, for loading ease, and burning at over 95% in the shorter tube, then Reloader 10X seems like the best compromise and it is a known good accuracy powder in the .223.

Your particular load may wind up needing more or less powder than predicted because of chamber tolerance differences. For example, the Lyman #49 manual has a 55 grain spire point seated all the way out at 2.26" COL (which should lower pressure as compared to 2.20"), but gives 25 grains of Varget as the maximum load using Remington cases. So, as with my two models, there seems to be a little lot-to-lot variation in Varget's burning speed or case capacity difference brand-to-brand or both working in concert.

If you decide to try RL15, Start at 20 grians and work up to an accuracy sweet spot. If you are sticking to Varget, try using a drop tube to load the cases, as that will pack the powder in better. Regardless of your powder choice, if you want a good systematic method of finding sweet spot loads, look at Dan Newberry's site.

As to actual velocities, you do need a chronograph to measure them with much resolution. You can build a ballistic pendulum or two separated disks spun by a synchronous motor if you have AC power at your firing line, but it's a lot of trouble to do either and the resolution is enough lower that you have to take and average a lot of readings to get to a reasonably precise result.

I've both read of and seen enough people having error trouble with the cheap chronographs that I'm not inclined to recommend any of them. You don't need a $750 PVM-21, but I think it is worth saving up your pennies for the CED M2 ($199 last time I looked). You can get a sense of how it compares to others on this thread at the Sniper's Hide. I also have an Oehler 35P (I set up two chronographs for velocity loss determinations to learn BC's). Like one of the posts in the SH thread, my CED and Oehler track almost perfectly.
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The velocity change per inch of barrel changes with the length of the barrel. The shorter it gets, the more you lose per inch. There are two reasons. One is that as the barrel gets shorter the pressure at the point of muzzle exit is higher and higher pressure accelerates more rapidly, so loosing an early inch cuts into acceleration more than losing a later inch, where the pressures are lower and more level. With BL-C(2) under a 55 grain bullet, if you are out at 24", then you see about 30 fps/in difference, but at 16" is is closer to 50 fps/in of barrel. That's why average seems high.

Another reason is the shorter the barrel gets, the bigger the percentage of barrel you are cutting off with one inch, so the bigger the portion of the total bullet acceleration you are eliminating.

Also, note that the .223 bullet is just plain going faster than a .30 cal. When you cut an inch off an inch from a 24" barrel for a bullet going 3200 fps, it causes a bigger loss in fps than making that same length change for a .30 cal bullet going 2700 fps.


BL-C(2) (cannister grade WC846) has been around for some time. It is a good powder, but like a lot of spherical propellants it is harder to ignite than stick powders and is more prone to erratic muzzle velocity as a result. At 100 yards, the resulting difference in vertical point of impact is almost invisible, but at 300 yards it tells a good bit more. You can see this by messing with muzzle velocities in your ballistics program to see the drop difference (the free online JBM calculators will do this for you, too). After subtracting the moa you got at 100 yards from the group at 300 yards, the remaining vertical dispersion is a good indicator of your velocity spread.

To get better consistency from BL-C(2), you can try working with magnum primers. You seem to be filling the cases pretty well, so that may not help, but it can be worth a try. Knock the loads down 5% and work up with a magnum primer. You can simultaneously run the Newberry round robin at 300 yards.

Making no other change, you can also try the benchrest technique of deburring flash holes. That helped the performance of my .308 loads using Accurate 2520 years ago, actually cutting my M1A"s group size a whopping 40%, from about 1.2 moa to about 0.7 moa. It never made any difference with any stick powder I used in that gun, but because of that experience, even though it usually makes little difference, I always give it a try with any new powder just to see if I get an improvement? Newberry eschews all the benchrest methods, but I find they help in a minority of cases. You have to take it gun by gun.
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To take it up from where MZ5 left off, some scopes also have a transition near the center of the adjustment range, either side of which the moa per click is slightly different. This is due to the inside barrel riding the leading edge of the adjustment post in one direction and its trailing edge in the other. Sightron boasted of their fix for this when they first came out with their line a little over a decade ago, and I have to say the good old mil-dot reticule on my copy has served me very well.

I appreciate the idea behind the circle reticule, but don't find it works as well for me as the mil-dot, either. I suspect the problem is that the eye is very good at centering something in a circle, but not so good at holding off center. People can turn in some very good groups even with ghost rings, holding the front sight post in the center of the blurry rear sight aperture, but I wouldn't give two cents for their chances of placing the front sight consistently at, say 4:00 in the blurry aperture and half a front sight post width from center, then doing it consistently enough to get a good group. It would be easier with the scope reticule, but that doesn't cover the inbetweens very well.

BTW, like compensating with mil-dot hold-offs, correct parallax settings become important to making that circle reticule work, too.
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I think that's usually true for all but benchrest 100 yard shooting, where muzzle velocity extreme spread has to be pretty gross to open a group up much. And that range covers most woods hunting in the East, where clear shots beyond that range are unusual. The game can't tell 100 fps difference one way or the other, at impact. At the other extreme, by the time you get to 1000 yards the 100 fps MV extreme spread that changes POI less than 1/4" at 100 yards can grow to a four foot difference in vertical POI, depending on the bullet and muzzle velocity. In that application you could be seriously short-changing your potential accuracy by not having a chronograph to help you develop more consistent loads. At the least, you'd be saving yourself a lot of load trial and error and unaccountable point losses.
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