A lot of water and dirt in M4 barrel, will that cause bore premature (premature explosion)?

Question

I got a lot of different answers from Firearms manufacturers, Mythbuster, and weapon masters. So, what will happen if you have a lof of dirt, water in your M4 barrel?
In Mythbuster TV show, they are using bolt action rifle to do the experiment and the front end of barrel is totally destroyed. How about M4, the automatic rifle?

Answer 1

I'm an armorer trained by Colt Firearms and the M-4 is one of the guns I work on.

The AR group (of which the M-4 is a member) works at extremely high pressure. Because the bullet travels at a fairly high speed, the bore MUST be free of obstructions to operate safely.

If you are using military specification ammunition (mil spec) then you are shooting a 55 grain, FMJ bullet that leaves the barrel in excess of 2200 feet per second. (That's approximate 1500 miles per hour). This bullet also develops chamber pressure in excess of 100,000 PSI (pounds per square inch)

Now imagine what happens when the bullet comes across "water" or worse, in the barrel? How does it overcome that obstruction?

To give you an idea of how "solid" water can become, when was the last time you did a belly flop in the swimming pool? How fast were you traveling when you hit the water? It was like landing on the ground if you jumped in from a high dive. Now imagine doing the same thing and hitting the water at Mach 2 (or faster). Something's gotta give.

The bullet can't go anywhere but straight, and because it's confined to the size and shape of the barrel it HAS to go straight. The water can't accelerate by itself to stay in front of the bullet, so the only thing left is for the barrel to "stretch" to accomodate both the water AND the bullet, often with catastrophic results. If you're lucky you will end up with a bulged barrel (where the bullet actually pushes the water to the side). If the barrel is plugged it will blow up (as in your Mythbuster TV show) to release the pressure created by the bullet. Mud moves even slower than water!!

Any gun that has an obstruction in the barrel will react the same way. A barrel plugged with a foreign object (be it a squibb, mud, water) will actually blow up to relieve the pressure, making the end look like an exploding cigar (often)

With that said, Colt has said that their barrels are of such high quality that they will NOT explode, but will buldge, and remain intact. You won't be able to shoot it again, but they say it won't fragment like other "lower quality" guns.

There are several M-4 knockoffs, only Colt makes this claim.

Answer 2

even a little bit of obtrusion can be harmful to your barrel. If it is "a lot" like you say, then it is quite possible--even likely--that the barrel will explode.

The 5.56 round is a very high pressure round, and is also quite small, making it very susceptible to obstruction problems.
BE CAREFUL!