It seemed like the Mk262 was designed to be used with the SPR.
Is there benefit gained from a 20” barrel with this ammunition, or was the round designed to reach peak velocity potential with the 18” barrel?
There will be a velocity increase when going from 18 to 20" with every .223/5.56 round I can think of. If we had an 18" barrel, I’d chrono the difference this weekend.
The difference in MV between a 18" and 20" barrel is pretty subtle. Typically in the 10-50 fps range if I remeber right. The Mk12 SPR is a 18" variant, and is used regularly for engagements in the 400-600 yard range, if that helps ya any. The only reason I’d own a 20" is because I’m kind of fond of the M16A2/A4. If I were building my own AR for any other reason though, I’d go with the 18".
I chrono’ed the difference between 16”, 18”, and 20” barrels with MK262. 16”-18” gained 20 FPS. 18” to 20” gained 15 FPS.
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With those very slight velocity increases, is that due to the powder mostly burning sooner?
The reason I ask is sourcing for a SPR-ish rifle. I use Mk262 in my 12.5”, and I plan to use the same for this rifle. I have my “fun guns” zeroed with cheaper stuff. So, an extra two inches wouldn’t bother me (that’s…), but if it isn’t going to have a benefit, I’d go 18”.
I have an 18" setup and the 20fps doesnt do much.
Hell, for that matter (according to the info redpillregret posted) the ~ 35fps total from a 16" to a 20" is rather negligible too. A well-balanced 16" barrel should be just about as good, “well-balanced” NOT meaning an A2 profile!
It made me give up my 18”. All barrels have variance but that was my results with two BCM SS barrels and a Colt 20”. I’ve heard of as much as a 50fps jump from 18”-20”…still not worth it for me. I’ll stick with 16”.
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There is a big difference between 18" and 20" barrels that is not widely known. Yes, you will get a little improvement in velocity from 18" to 20," but more importantly, the pressure and dwell time on a 5.56 NATO round is tuned for various pressures, gas port sizes and locations, and distances (most importantly) from gas port to muzzle.
20" with a rifle length gas port is optimal. Works well for many, many years. The 18" presents a pressure imbalance that results in premature gas port erosion (read: increase the gas port size and gas flow over time). We are talking about after a few thousand rounds, so it may or may not affect everyone, but this is well documented.
For the Navy’s SPR, which become the Mk12 sniper rifle in 5.56mm had/has a 18" Douglas SS barrel. For some reason, the Douglas barrel has performed much better than other blanks in 18." Better than some other more well thought of top-notch barrel blank makers. It must be in the steel that Douglas uses.
Thus, for weapons designers that we work with, and customers, I recommend a 20" barrel over 18," all other things being equal. For those who want an 18" barrel, I recommend using a Douglas blank, unless your overall shooting will be of relatively smaller numbers, as the erosion will not take place right away. It is a high-volume shooting issue. And, keep in-mind, that a barrel is a normal wear item, like brake pads and rotors on a car, so you can always just replace the barrel when wear starts to show.
I don’t believe Douglas uses any formula for steel different than other manufacturers. Wasn’t the MK262 round designed for the MK12 specifically?
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Yes, the Mk262 was designed for the SPR (Mk12). Yes, the Mk262 performs great in many rifles, including 10.3" Mk18.
I am not privy to their steel, but I know enough about barrel construction to say that the actual steel is not the same barrel maker to barrel maker. I have seen the barrel damage, one barrel blank manufacturer to another, inside the bore at the gas port, and the Douglas barrel has just worn less and lasted longer. This has been documented. I assume it is the steel. I am not sure what else it could be.
There was a guy on another forum who ran tests on barrels/finishes years back. He noted that certain powder (burn rates) and bullet combos made drastic differences in port erosion.
I’ve never found port erosion was anything to worry about for the average shooter. Perhaps if you do a LOT of training and high volume shooting, it might be a concern.
As I understand it, Douglas, as well as the other major barrel manufacturers purchase cylindrical rods of a given formula from an outside source, no matter if it’s Bartlein, Douglas, Kreiger, etc. Barrels are chosen based on the desired formula of 4140, 4150, 416, 416R, etc.
In most instances, a 4140 blank from Douglas will be of the same 4140 from the next maker. The steel follows a given formula or isn’t given the label.
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Is the gas port on the Douglas initially sized a little differently, as an alternative explanation for the difference in wear?
I find this very interesting and would love to see a clear reason, especially if we’re seeing differences from manufacturer to manufacturer, when using the same steel.
The gas port thing seems like much ado about nothing for an 18-20 inch barrel.
I’m not seeing how, all things being equal, an 18" rifle gassed barrel should wear faster at the gas port than a 20". The 18" barrel will have a slightly shorter duration of gas impulse because of the shorter distance from the port to the muzzle. In either case the port is far enough from the chamber that port pressures and the associated wear are relatively low (and nearly the same for 18" & 20").
Short barrels with their shorter gas systems are really where gas port erosion becomes an issue and can greatly reduce barrel life.
Barrel material and surface treatment should have an effect, but as far as I know Douglas uses 416 Stainless for their AR15 (SPR, Recce, Service Rifle, etc) barrels, or at least that’s what I remember from eyeballing them on Compass Lake Engineering’s website. 416 is widely used by barrel makers and isn’t something specific to Douglas.
Tx you are correct on 416 in the Douglas. Believe it or not, it’s a bit of an outdated choice these days. 416R or 410 may have better wear properties.
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The MK262 does not work well in 10.3" set ups. I have seen it break more Bolts than you can think of. I do not use it in anything shorter than 12.5". That is one of the main reasons John Noveske was working on the 12.5" barrel.
OP- Nothing really gained from 18" to 20" with the MK262. It was designed to be used in the MK12, 18" rifle gas system.
As far as velocity with 18 vs 20 inch barrels, in my experience, the difference is negligible.
Rule of thumb is 25fps per inch.
However, barrels of the same length can vary by 100fps.
YMMV, not valid with other offers, void where prohibited.
That is really not the case. There is a great deal of variability in the steel quality, and the making of a great barrel begins with the steel supplier, then continues to the barrel driller or blank manufacturer and then to reamer (chamberer) and lathe shop. Each step has its degree of variability and differences. Douglas is not typically known as the best of the best, but they are darn good, and it just so happens, that empirical evidences suggests that 18" stainless barrels do better from Douglas than other shops. I am not smart enough to know what is different, but many hypothesize that it is the underlying steel alloy. It is possible that it is something else, but I cannot imagine what if might be, if not the metal itself.
Either way, this is after 2,000 to 6,000 rounds of firing, so it could very well be a discussion of how many angels can dance on the head of a pin, for most recreational shooters.