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Posted: 6/10/2009 4:36:34 PM EDT
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Rifle: Armalite AR15(T) 16 inch
Optics: ACOG Brass: PMC brass dillon swaged once fired Full Length Resized Trimmed to 1.75 on Giraud trimmer Flash Holes Deburred Primer Pockets Cleaned Primer: CCI 400 SR Powder: 23.9 gr. H-335 (I'm trying to match the ballistics built into the ACOG) Bullet: Nosler 50 gr. Ballistic Tip COAL: 2.26 Loaded on Hornady L-n-L AP. Powder charge seems consistent beginning to end, but I did not weigh each round. Ten shoot groups under one inch at 100 yards. With different optics and a different rifle, this load shoots ten shot groups down to .440. So what's the problem? This load chronographs ten rounds from 2650 to 2982 fps for an average of 2725 and standard deviation of 107. If I throw out the two outliers at the high end, I'm left with 2650 to 2693 for average 2677 and standard deviation of 17. What could be causing the two outliers at over 2900 fps when the others are very nicely grouped around 2677? The outliers don't seen to be turning up as flyers at 100, but I bet they will at 500. Help, please. |
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You complain about your two shots at +2900 fps (I assume you figure that is the reason for the 10 shot, 1" group)............jeez.
OK............weigh out every case, measure every case for trim length, weigh out every bullet, measure out every bullet for length, check every bullet for run out, check every loaded cartridge for run out, check every bullet's profile. Then, placement of the round in relationship to the chamber can also affect things. Did I miss anything? Some reading...... http://www.6mmbr.com/jgcaseprep.html http://www.larrywillis.com/bullet-shape.html OH..........you were just bragging.........right. 
Aloha, Mark |
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OH..........you were just bragging.........right.
Aloha, Mark I don't think one inch groups are anything to brag about among this group. I am puzzled however why two rounds would be at 2900 when the other eight were at 2670. I repeated the experiment twice from the same lot, and got the same results but not as extreme the second time. You gave me some reading to do and I will do that, but is this normal? Any other insights and suggestions would be appreciated. Thanks in advance. |
| It is hard to say just what is the problem. Most likely the measure threw a high charge for those rounds. You said the second string was better. The only way to know is to weight a string of say 20 throws to see just how consistent the charge is throwing. Do it just as if you were loading or it may be different. I would not weigh each case, bullet or all the other crap unless you are going for extreme accuracy. It could be heavier than normal cases with good charges. Pick up your cases after each string and weigh them if they throw extra high velocities. |
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Thank you river_rat for a good lead. I have not checked the consistency of the powder drop since H-335 is flowing like water, but will do so. I am not looking for extreme accuracy. One inch from an AR-15 is good to go for my purposes. I'm just puzzled by the extreme velocity variance of two rounds in an otherwise very consistent velocity group.
If the powder drop is not the problem, it might well be case capacity as We-rBorg suggests. I will identify the high velocity rounds and see if the cases are out of the normal range. If that's the problem, PMC brass is headed for the CQB training bucket. Magnum primers are a new idea for me. Are many reloaders here using magnum primers? I assume I should drop the powder charge. Would 10% leave me in the same ball park? Finally, what standard deviation do you consider normal or an attainable goal? I realize what counts is consistency at the terminal end, but a standard deviation of over 100 doesn't seem right. All great leads. Many thanks. |
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My best AR-15 200 yard group to date was shot with PMC cases. I was actually shooting them as foulers, after changing loads, prior to testing a load in my "good" LC cases. The LC's performed well, but still the PMC groups were the best group for that load.
The experience reinforced in my mind the idea that small changes in stuff like case capacity (or headstamp) can influence accuracy for some loads in some guns. Load was H-335/CCI 400's/Nosler 60's. |
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Powder drops were inconsistent. Looking like a dirty powder dispenser is the problem. Running fine now. Dead on. I won't know for sure until I shoot it, but powder charges seem to have at least part of the problem. River_rat nailed it in one. Thank you.
I did weigh 20 prepared PMC cases. Here's the breakdown: Weight Range (gr) / Number of Cases 92.6 / two 92.7 to 93.0 / nine 93 to 93.5 / three 93.5 to 94 / two 94 to 94.5 / three 95.3 / one That looks like a pretty wide distribution to me. What is normal? |
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Forget about weighing cases and bullets or checking the run-out of anything. It’s all irrelevant to the huge variations in velocity that you are getting. Velocity variations that large usually boil down to the powder charge and its ignition.
As you’ve already discovered, your powder charges are inconsistent. With ball-powder and a quality powder measure you should have no trouble obtaining a variation of plus or minus 0.2 grains and most likely better. Another big concern is the volume of the available case capacity that your nominal powder charge fills. Your 23.9 grains of H335 only fills approximately 86% of your available case capacity; that’s terrible for a match grade load. I won’t even consider using a charge that doesn’t approach a 95% fill ratio for match grade ammunition. It’s a well established phenomenon that lower percentage fill capacities allow the powder to reposition itself in the case differently from round to round and result in different ignition patterns and large variations in velocities. A standard deviation of 12 FPS for a 10-shot string is considered the “Golden Standard” for match grade ammunition. The Crane Naval Surface Warfare Center requires a standard deviation of 15 FPS for a 10-shot string for ammunition intended for precision work. |
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Good reply and advice Molon. I will try again as soon as the rain stops here to see if the more consistent powder charge makes a difference. It should. I had not thought about fill volume. I'm trying to match the ballistics in the ACOG with this load which is on the mild side and therefore a low fill rate. I don't need better than one moa accuracy with this rifle, but you have given me something to think about and work on. I may need to go to a heavier bullet or a different powder to achieve that goal.
In the medical field, we say, "If you hear hoof beats, don't think of zebras." I like your advice a lot. Many thanks. |
A low standard deviation by no means guarantees that a load will be an accurate one. It does however show that you are doing things consistently correct in your loading process; consistent powder charges, consistent neck tension, consistent case capacities, consistent seating depth, consistent ignition pattern etc. Consistency is one of the fundamentals of an accurate load. The chronograph print-out below shows what is possible for a standard deviation for a 10-shot string of a hand-load fired from an AR-15; in this case a 77 grain Nosler fired from a 20” barreled AR-15.
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Quoted:
A low standard deviation by no means guarantees that a load will be an accurate one. It does however show that you are doing things consistently correct in your loading process; consistent powder charges, consistent neck tension, consistent case capacities, consistent seating depth, consistent ignition pattern etc. Consistency is one of the fundamentals of an accurate load. The chronograph print-out below shows what is possible for a standard deviation for a 10-shot string of a hand-load fired from an AR-15; in this case a 77 grain Nosler fired from a 20” barreled AR-15. http://www.box.net/shared/static/rvg2fm47r9.jpg You may want to check the screen distances on the Oehler 35P. Your proof velocities are pretty far off from the instrumental. When I used one, I had them on a piece of conduit and the location dimples were made so the main screens were 4ft (+/- 1/16") apart and the proof screen was centered within 1/32-1/16". My proof velocities were normally within 1 or 2 fps of main screen velocity. |
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Quoted:
Quoted:
A low standard deviation by no means guarantees that a load will be an accurate one. It does however show that you are doing things consistently correct in your loading process; consistent powder charges, consistent neck tension, consistent case capacities, consistent seating depth, consistent ignition pattern etc. Consistency is one of the fundamentals of an accurate load. The chronograph print-out below shows what is possible for a standard deviation for a 10-shot string of a hand-load fired from an AR-15; in this case a 77 grain Nosler fired from a 20” barreled AR-15. http://www.box.net/shared/static/rvg2fm47r9.jpg You may want to check the screen distances on the Oehler 35P. Your proof velocities are pretty far off from the instrumental. When I used one, I had them on a piece of conduit and the location dimples were made so the main screens were 4ft (+/- 1/16") apart and the proof screen was centered within 1/32-1/16". My proof velocities were normally within 1 or 2 fps of main screen velocity. My sky-screens are mounted on a 4' foot rail purchased directly from Oehler, dimpled by them and measured by myself to confirm placement. The differences between the proof velocities and the primary velocities are well within the statistical parameters set-up by Oehler. If they weren't, the microprocessor would have flagged them with asterisks and they would have been omitted from the string. An average of 5.9 fps difference between the proof velocities and the primary velocities, for a load with a mean velocity of 2639 fps, is a variance of only 0.2%, which is hardly “pretty far off.” |
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