SIG MPX 9mm barrel plunk test - strange results?

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SIG MPX 9mm barrel plunk test - strange results?

Post by jlow on 1/8/2019, 1:37 pm

Thought I would give it a try.  

I have two OEM barrels - a 4.5" and a 8".  Both relatively new less than 1k rounds.  Tried my normal COAL case (1.145") with Precision Delta 147 grain FMJ, case length 0.745", passed plunk test.  

Thought I would try a really long one to start - COAL 1.2565 - passed???  I actually pressed the case in with my thumb to make sure it was seated, plunk right out.

Did not do the rotation test as this is a rifle barrel for a rotating bolt i.e. like an AR so the round is about 1/2 deep in the chamber.

Is this normal?

jlow

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Re: SIG MPX 9mm barrel plunk test - strange results?

Post by jlow on 1/8/2019, 1:59 pm

One thing I can add is since I cleaned the barrels before doing the plunk test and the barrels are off the gun, I could look inside from the chamber end with a magnifier.

What I noticed is the barrels are in excellent shape but the lands where they start at the chamber end appears to have a short 1/4 inch ramp?

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Re: SIG MPX 9mm barrel plunk test - strange results?

Post by Jon Eulette on 1/8/2019, 2:14 pm

If not mistaken the MPX is sub gun type weapon. The chamber throat is longer to allow higher pressure 9mm cartridges to bleed off pressure by allowing gas to blow past the bullet during firing. Back in my MP5 days I remember there was ammunition in FRG that was designed for sub guns and was hotter than the pistol ammunition.
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Re: SIG MPX 9mm barrel plunk test - strange results?

Post by jlow on 1/8/2019, 2:21 pm

Yes, the MPX is a sub gun type.  Thanks for that info, it is useful to know - much appreciated.

So that would mean the bullet will have a big jump and there is no reason to do a plunk test.

Would that also mean there is likely a limit to their precision because of the jump?  In centerfire rifles, a long jump usually is not ideal although not always terrible.

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Re: SIG MPX 9mm barrel plunk test - strange results?

Post by jlow on 1/8/2019, 2:26 pm

I see that you are a former AMU Pistol team member.  What is your thought on trimming pistol brass to obtain uniform COAL as per the "Precision Handloading for Pistols — Tips from the USAMU" on Accurateshooter.com website.

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Re: SIG MPX 9mm barrel plunk test - strange results?

Post by Jon Eulette on 1/8/2019, 3:35 pm

Don’t waste your time trimming brass. Time is better spent dry firing and shooting. I’m getting 1.75” groups with mixed brass reloads and am calling all my shots. You could use new and once fired brass for 50 yds and crap brass for the short line. Better brass will get you down to 1.25” range.....is it worth it? I only use newer brass for bigger matches. If you are shooting 2650’s then you might consider better ammunition.
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Re: SIG MPX 9mm barrel plunk test - strange results?

Post by jlow on 1/8/2019, 4:34 pm

Thanks!  But I think if I can get even 2" groups with single headstamp brass, I would be happy to just practice shooting.  The reality is under your conditions, I am getting more like 4-5" groups at 50 yards....

BTW, it's not my shooting skills as I can shoot single hole groups at that range with a centerfire rifle.  I have my MPX on an expensive bipod, a 10x scope and a bag in the rear.

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Re: SIG MPX 9mm barrel plunk test - strange results?

Post by SteveT on 1/8/2019, 6:26 pm

A 4-5" gun will get you to Expert and even Master. You won't even notice until you can call your shots within an inch or so at 50 yards, then you'll occasionally have shots that make you go hmmm. Remember that a 5" gun will shoot all shots within 2.5" of where the gun was pointed and half or more of the shots will be within 1" of where the gun was pointed. If the shooter can pull the trigger when the gun is pointed at the black, they will score upper 80's to lower 90's at the long line and mid to upper 90's at the short line.

I often shoot better scores with my original ~4" wad gun than I do with my super wizbang custom built sub-2" Rock River Arms. It frustrates the hell out of me.
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Re: SIG MPX 9mm barrel plunk test - strange results?

Post by jlow on 1/9/2019, 8:04 am

What you say might be true, and I appreciate your input, but I should clarify and say that my gun is not going to be used for bullseye shooting but 2-gun matches.  Where I am having problems is shooting small steel poppers 2" across at 50 yards and 8" steel plates at 100 yards.  I can hit them but it takes more than one shot and that burns time.

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