What does this data tell me?


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CMV
December 26, 2011, 08:12 PM
Found a 100 yd outdoor range about an hour away & spent a few hours there today. I fired my first 30 reloads. Strings of 5 at each powder charge. Now that I have the Chrony data and groups I don't know what to make of it.

I started with XM193 and IMI M193 to get the Chrony set up and to sight in the cheapo carry handle scope I just got. Then I fired my strings. I stopped after each one to collect & inspect the brass.

They fed, fired, cycled the weapon, extracted, etc with no issues. I noticed slightly less recoil with my loads compared to the factory M193 but recoil is already pretty negligible with an A2 buttstock & full size buffer & spring so that might have been my imagination. I saw no signs of anything wrong like cracked/split cases, flattened primers. The only thing I noticed any different was that as the charges went up, the extracted brass flew out a little further.

AR-15, 5.56mm chamber, 20" 1:9 HBAR, 53-55, sunny, mild to no wind. Chrony F-1 10' from muzzle per its instructions. LC brass, Hodgdon H335, CCI #41, Hornady 55gr FMJBT. Shooting from a bench with a sandbag.

http://cmv.zftp.com/RLdata.jpg


Are 5 shot strings enough data points to form any conclusion? The IMI & XM193 were only 3 shots measured for velocity due to some initial Chrony errors so their std dev might be skewed because there are fewer data points?

My group sizes are all good for me. The cross hairs on the scope completely cover the target so I was actually surprised they were that tight. I think that's why the last target grouped a little better - I could see some of it. I should have used larger targets, but with the 25M zero targets I could put them all on the backstop & not have to keep waiting to go cold & walking back & forth (lazy). I never used a 4x20 scope before & didn't realize how weak it would be until I was looking at these little guys through it. I don't have the factory load targets pictured below but they were at the bigger targets too. I think they'd have spread more if I was trying to hit the zero targets.

Target #1 was 23.5 gr, #2 was 23.8, etc.

http://cmv.zftp.com/IMGP2427.JPG

Anyway, I'm kind of lost as to what this tells me. Other than velocity, it seems any of the powder charges were just fine. The larger/smaller groups could be as much my fault as the ammo's fault. Interesting, there was no change in elevation - they all pretty much hit in the same place.

I'm also only .3 gr away from max load in my book (Lee - Modern Reloading 2nd ed) and nowhere close the the factory velocity or the published 3,203 in the book. It doesn't say what bbl length, case, or primer was used for testing, but from a 20" shouldn't I be closer? at 1.7gr over the starting load is where I'm matching the starting load velocity. Looking at how the velocity is creeping up past 24.1gr, I would estimate at max load I'd be at 3,050fps.

http://cmv.zftp.com/loaddata.jpg


If anything, I thought I'd be a little higher using 5.56mm brass. The interwebs told me I'd build up more case pressure due to thicker case walls and I thought that would translate to slightly higher fps. I don't really care about velocity - I'm not making these rounds for zombies so if they fragment or not when they hit the backstop doesn't matter. What I care about is is I have a reference telling me I should be seeing X and I'm seeing (X - a lot) indicating I'm doing something wrong. Per Federal website, XM193 should be at 3290 at the muzzle and I'm only -20 from that (which may be from the 10' distance?). So that tells me the Chrony is right and I'm seeing what I should from factory loads, but not mine.

I think I'll need a better scope for one thing. My eyes aren't good enough for iron sights - nor are my shooting skills. I'll hit a B27 all day long but will be into the 9 ring in all directions with iron sights @ 100 yds. Wouldn't get tight enough groups to tell me much. But I didn't even try today. I'm so spoiled by the Aimpoint I haven't even used iron sights in a couple years. If the zombies rise & my battery dies I'm in trouble :)

I measured each powder charge twice on 2 different scales but I still saw a lot of variation from identical loads. the difference from highest to lowest fps for each charge weight:

23.5 - 112fps spread
23.8 - 77 fps spread
24.1 - 56 fps spread
24.4 - 77 fps spread
24.7 - 141 fps spread (2 out of this group were faster than the 25.0 average)
25.0 - 38 fps spread

So my overall conclusion is I need to play around closer to 25.0 - maybe 24.8 - 25.3 in .1 increments. I saw the tightest group, lowest SD, & most consistent velocity at 25.0. But my methods may be so flawed that I'm basing that off useless data.

Side note: it felt so weird looking for my brass! So used to just leaving it lay. Was like being back in the Army policing up brass :) But if I fired 5, I hunted until I found 5. And some kind soul was nice enough to leave me about 100 pieces of 1x fired LC brass around my table so if nothing else I got 100 free pieces of brass today :)

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rsrocket1
December 26, 2011, 09:19 PM
It looks like you want 25.0g H335 for the 55g pills. That's my go to load for my 20" AR also. I would focus on the groups more than the SD, especially for the commercial loads. SD for 3 shots is worthless, just like a shot spread for 3 shots is not a valid test.

If you go back and shoot several 10 shot strings with 25.0g H335 and got the same results, you are good to go.

Note that H335 is somewhat temperature sensitive, so your results may be a little different next summer. You can also see that even with a 0.6g difference, your groups aren't changing dramatically. That is a pretty good indication of a node.

Your next test might also consist of variable loads in that "node" area to see how well you can hold that group. Again if your group is acceptable at +/- 0.3g, you are really in a good spot.

Before you know it, you'll be looking for a way to catch that brass in a net or brass catcher!

denton
December 26, 2011, 10:14 PM
Lake City brass tends to have more case capacity than commercial brass (reverse of some other chamberings). That will get you lower pressures and muzzle velocities. That in turn may partially explain why you're getting lower MVs than IMI ammunition. The other part of the equation is that the military seems to run their loads a little hotter than SAAMI specs.

At the target, there isn't a huge difference between 3050 FPS and 3200 FPS. I generally run at around 3100 FPS, and feel fine about it.

Yes, you can draw conclusions with 5 shots at each loading. But to do it right, someone analyzing the data would need all of it, not just the summaries at each charge.

Pick what you like in the range you've explored.

CMV
December 26, 2011, 11:27 PM
OK, so I loaded up 30 more for testing tomorrow.

10 ea of 24.8, 24.9, & 25.1. I expect to see some overlap since they're all so close.

Was very careful with every step. Trimmed & measured each case to within .0015 spread, then seated primers, then measured each charge and weighed it again on another scale.

I did find two possible causes of my variations - both dealing with powder measuring.

1. I'm filling the cases with an RCBS funnel. when I take the case out of the funnel a little powder spills out. The neck of it is just a little too big even though it's the "22" one in the kit. The funnel spout needs to go into the mouth instead of around it. Anyway, every spec that spilled (just about) got swept up & put back in.

2. I didn't realize it before, but how the powder is placed in the tray for the Lee safety scale makes a big difference. If it's heaped toward the front or the rear instead of evenly dispersed it can be off by 2/10. I use a Hornady auto charge scale & dispenser. Whatever I set it for, I lock the little Lee scale to that weight & then double check the Hornady that way. So if the Lee wasn't balancing right at the mark, I was adjusting rather than trusting the electronic scale. I'm going to guess that weighing on the verification with the powder not even in the basket led me to add/remove powder when I shouldn't have & that's why I ended up with things like 2 rounds loaded at 24.7 going faster than all the rounds loaded at 25.0. So if the Lee told me the Hornady was off, I tapped the basket to even out the powder & then reweighed. Most times I had the same results, but a couple were spot on after doing that.

I didn't try to chase OAL measuring at the tips. They're all longer than 2.20 - beyond that I didn't fool with where my seating die placed the bullet this time.

I'm hoping these things will give me more consistency tomorrow. I doubt it will effect grouping or POI because I'm the lowest common denominator in the equation.

If I still have these variances I'll weigh & mic each bullet next time to see if that's where it's coming from. Beyond that, I really don't know what to do differently.

noylj
December 26, 2011, 11:58 PM
1) You appear to once again have discovered that a small std. dev. and a small group do not always/seldom/never go together.
2) You need to know your case capacity and the case capacity of the tested rounds
3) You need to know your chamber and barrel dimensions and the chamber and barrel dimensions of the barrel used for the manual data
4) Small targets often produce smaller groups as the shooter concentrates more
5) With group sizes like that, there can't be that much more to improve
6) A new barrel will almost always have "machining" marks and should be thoroughly cleaned frequently for about 100 rounds.
7) Now, are you a reloader or a shooter? I'm a reloader and I would spend my time chasing smaller groups. A shooter would try for smaller groups by learning to shoot better. You are on the pinnacle and could fall either way.

denton
December 27, 2011, 12:16 AM
Before you end up getting yourself a hernia:

Most of the commercial ammunition I have tested tends to have MV standard deviations in the neighborhood of 35 FPS. Statistically comparing standard deviations typically requires samples much larger than those needed to compare means. Ignoring the commercial ammo, there is nothing in your data that indicates cause for concern on consistency.

It's not hard to get your SD down around 20 FPS, and with a little effort you can get it into the low teens. I have done this, and know of no good reason to do it again.

W.E.G.
December 27, 2011, 12:36 AM
Expect velocities from ammo fired in an AR-15 to be about 100 fps slower than predicted velocities in loading manuals.

The AR has an extra hole in the barrel you know.

The 100 fps thing is something I have noticed from several chrono sessions.

Factory ammo specs are usually even worse as far as being over-estimated when compared to AR-15 velocities.

gamestalker
December 27, 2011, 01:45 PM
A quicker method of making a determination would be to run those loads through a B.A.. If your velocities are still spreading out that much, it likely has something to do with the load and, or, components. But my gut feeling is your AR-15 20" is doing what they do. And the groups look good for the platform they are being fired from.

CMV
December 27, 2011, 09:13 PM
BA?

wanderinwalker
December 27, 2011, 09:39 PM
Congratulations, you've discovered why I refuse to weigh and trickle charges for my AR-15 (and this is making match ammo to shoot to 600 yards!). No matter what, there is going to be some deviation just from the fact that you are tapping gas to run the action. I set my powder measure to throw the corect charge weight, verify the setting over 2-3 throws on the scale and commence to measuring, checking one of every 10-20 throws.

Your numbers would be telling me the powder is working better at the higher pressure of the heavier charges. Honestly, I think loading at 24.8, 24.9 and 25.1 is entirely too close to determine anything useful. At .1gr variation, in the .223 you're talking about maybe 10-fps average, which you can see doesn't mean anything when your extreme spread is in the 30-fps range (or more).

My suggestion would be to set your meter for 25.0gr and load 10-20 rounds. Don't weigh each charge, just check it after say 10 rounds. Pay attention to how you operate the measure. If something feels different, toss that charge back in the hopper and cycle another one. Take them out and see how they shoot.

And don't stress over velocity too much. You're in the ballpark if you're within 100-150 fps. I've had people tell me how hard you HAVE to drive 600-yard ammo in the .223, yet none of my loads has ever had a prayer of going much faster than about 2650-fps or. Guess what? Yep, they shot pretty good. At worst 100-fps is an extra couple of 1/4-MOA clicks on a windy day.

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