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Posted: 4/25/2005 9:56:55 AM EDT
| I am going to have a 16" M4 barrel cut down for an SBR project. I want to go as short as possible (like 10") but am interested to know how much velocity will be affected by subtle changes in this barrel length range. It would really be useful to see some data on M193 muzzle velocity versus barrel length in the 10-12" range (say every 0.25"). Obviously, a crude curve could be extrapolated from 16"/14.5"/11.5" velocities, but it would be great if someone had data with more resolution. I guess I'm trying to understand if there is a "sweet spot" I should cut to. |
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From the Ammo Oracle: www.ammo-oracle.com/body.htm#fragrange Lists range to 2700 FPS for XM193 & XM855 in 20", 16", 14.5", 11.5" bbls. |
50 yards is nil? You must have people you consider threats at over 50 yards a lot. You must have a huge house and yard. How do you determine if somone is a threat to you in civilian life at over 50 yards?
This is 100% not true. |
To the Oracle for some reading with you, my friend. 50 yards out you're still above 2700 for XM193, the "magic" number; that's at 11.5" bbl I'm afraid any shorter than 11.5 and yes, you are just punching holes. In my house 50 yards is plenty...... That said, I use 75gr Hornady TAP for SD, but 193 would work at close distance. |
The ammo oracle data referenced above can be used to create a chart like this: Unfortunately, this data is not quite what I am looking for. For one thing, there is no data below 11.5" and so the chart relies on a dubious extrapolation in my range of interest. Moreover, the data shows a decided "kink"; this could be an experimental anomaly, an artifact of the ballistic calculator used, or it could be a genuine representation of the ammo's behavior. What I really want is the muzzle velocity data with which to create a "bang-for-your-inch" chart... ideally, with velocity data at every inch. Can anyone point me in the direction of such data ? |
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Here is some work I did a few years ago. The temp was 85 degrees, low humidity, elevation 1410 feet. Ten shot groups with close screen 10' from bbl. Barrel length - 20", 16", 14.5", 11.5", 10.25" LC M193 - 3255, 3145, 3075, 2884, 2767 Q3131A - 3152, 3051, 2951, 2732, 2605 IMI M193 - 3131, 3032, 2917, 2726, 2598 REM UMC - 3122, 3001, 2878, 2705, 2499 |
Tom, YOU DA MAN !!! Thats what I'm talking about. Here's a chart for LC M193: Interestingly, Tom32's data shows that even the 10.25" barrel throws M193 out above the 100% frag threshold of 2700fps. Moreover, Brou's reports that fragmentation can occur as low as 2300fps may imply a sporting chance of effective terminal peformance even out of the super-short 7.5" barrels (which, by this extrapolation, could yield around 2500fps at the muzzle). If I can find the time today, I may try to augment Tom32's data with some ballistic numbers and project effective frag range for each barrel length. Watch this space. |
It would be exceedingly rare for an M193 bullet to fragment at all at 2300 fps. They rarely fragment at 2500 fps, and don't reach maximum fragmentation reliability (which seems to be around 90-95%, not 100%) until right around 2700 fps. The reason it doesn't reach 100% is due to variences in bullet construction that are common with mass-produced FMJ bullets. 75-77gr bullets fragment down to 2300 fps, but they also start off much slower, and generally require a 1:7 barrel (this is true much more often on barrels shorter than 14.5"). Given the above, I would NOT count on fragmentation at any more than across-the-room range from an 11.5" barrel using M193, and anything shorter can just about forget it entirely. There's a reason why the folks with 10.5" barrels in Iraq use Mk262 (77gr OTM) whenever possible. -Troy |
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Thanks Troy - point taken. Honestly, I'm making the SBR for shits and giggles only - I have a 16" if I ever needed to "get serious". Do you know of anywhere I could find data on % fragmentation vs velocity ? Thinking about this subject, however, I am interested in understanding the relationship between barrel length and velocity in more detail for other ammo. Once my SBR paperwork comes in, maybe I'll collect some different types of ammo and run it over the chrono as I shorten the barrel an inch at a time... it will be neat to have a dataset with better resolution than the usual 10.25"/11.5"/14.5" barrel lengths. Watch this space. 
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Make sure you understand the operative word there. It CAN happen. Not likely, but I've seen it once. It wasn't spectacular either (but better than no fragmentation). I just don't want people to come to the wrong conclusions. |
There's a Fackler pic in the Oracle where M193 was tested at a bunch of different impact velocites, and pictures of the bullet fragments were shown. Unfortunately, this type of testing is extremely tedious and very expensive, so it just isn't available for most rounds. I would estimate that replicating the data in the pic I'm referring to today would cost somewhere around $20,000-$30,000, due to the high cost of gelatin and the failure rate of the gelatin blocks (if they don't calibrate correctly, they can't be used, and they cost about $60 apiece just for the gelatin powder). Of course, I know where you could have this testing done if you have the skrilla...  -Troy |
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By ar15.com's own simply awesome Ammunition forum gurus: archive.ar15.com/forums/topic.html?b=3&f=16&t=209674 |
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