27 May 2014

Cutting HK MR223/MR556 barrel - good, bad or ugly?

There were several discussion on cutting and re-profiling 16.5" heavy match barrel that HK installs in MR223 and MR556 series rifles. Profile is rather mater of personal choice and taste, but cutting down generates some controversy. HK claims that when they cold hammer forge their barrels, they make them in way that barrel bore has a swage (constricts) toward muzzle. Barrel is manufactured for it's final length, so cutting it down remove this narrower portion and affects barrel performance. I decided to find out how it turns out in real world.
Last year I cut my MR223 barrel down to 14.5" with 79dgr target crown and slightly re profiled it to loose some weight. I went with something close to SPR profile.

First question is obviously for accuracy. I'm not big on shooting for accuracy. I do not even have a good rest for rifle, but usually I can improvise something or use sandbag that is laying around on the range. Due to problem with access for good ranges, when I zero my rifles I usually shoot on 50m range, sometimes on 100m one. Before cutting down my barrel, my practical accuracy with quality ammo for 5 shot groups with 4x scope was from 0.966MOA (100m, Hornady 75gr HPBT Match) to 1.348MOA (100m, RWS Target Elite plus 69gr HPBT). After I cut barrel down I shot some groups at 48m (little over 50yrd) - max my club range allows for rifles. With same 4x scope I registered 5 shot groups of 0.731MOA and 1.084MOA (Hornady Steel Match 75gr HPBT), then 0.827MOA and 0.978MOA (RWS Target Elite plus 69gr HPBT). So basically, no. Cutting barrel did not negativelly affect practical accuracy of my MR223 rifle.

What about bullet speed then? Obviously, going down from 16.5" to 14.5" did negatively affect V0 value for my rifle. For my mostly used ammo (Hornady Steel Match 55gr FMJ) V0 went down from some 874m/s to 824m/s (2867fps to 2703fps). I was little disappointed to see 82fps loss per inch of barrel. I was expecting to be in range of 50-60fps/inch speed loss. But this did not answer question, how things would work with factory 14.5" barrel. I could not test that with HK416, first because I do not have HK416 on hand (but I could get access to one for few shots for sure). But second and more important reason is that HK416 barrel is chrome lined barrel. So it is not same beast as non-lined MR223 match barrel. It would be like comparing apples and oranges. But this changed.

For some time HK is selling in Europe factory MR223A1 rifle with 14.5" heavy match barrel. One of my friend have such rifle. All it took was for us both to meet at the range with our 14.5" rifles, chrono and few rounds of ammo. This happened last Sunday, on 3Gun match both of us attended.

I took my Pro Chrono Digital and shot three 5 rounds string from each rifle in exactly same conditions and from exactly same distance from chrono (around 15ft). Ammo used for this test was 2 kinds of 55gr ammo - Hornady Steel Match FMJ (that I shoot) and Aguila FMJ (that my friend uses) and surplus 62gr FMJ spanish SB ammo (that I have some left). Results are interesting.

Hornady Steel Match 55gr FMJ ammo averaged from my rifle 828.80m/s (2719,16fps - please note that fps values are calculated from m/s values measured with 1m/s accuracy). Factory rifle averaged at 834.20m/s (2736,88fps). That gives difference of 5.40m/s (17.72fps) between rifles.

Aguila 55gr FMJ registered respectively 838.40m/s and 840.80m/s (2750.66fps and 2758,53fps). In this case difference was smaller at only 2.40ms (7.87fps).

Last ammo tested was Spanish surplus SB 62gr FMJ ammo. In this case measured velocities were 827.60m/s and 833m/s (2715.22fps and 2732.94fps). For heavier ammo V0 difference was ...... exactly same as for Hornady Steel Match - 5.40m/s (17.72fps). That was a surprise, but most probably also a coincidence.

Final conclusion is, that HK seem to be right with claims that factory 14.5" barrel will show higher muzzle velocity that 16.5" barrel cut down to length. However difference is so small 2.40-5.40m/s (7.87-17.72fps). This is 0.29% to 0.64% muzzle velocity loss. In practical use it is negligible. I also did not notice any practical accuracy loss (on my level of skill and for ammo used), that would be important for distances I shoot (up to 300m, mostly within 100m). I'm going to enjoy my cut down barrel until it will be shot up and then I will replace it with... factory 14.5" barrel - to save on cutting, threading and crowning cost, while enjoying HK warranty on barrel life.

Below is raw data for your enjoyment. Data was collected in m/s. Fps values calculated for convenience.


M/S

Hornady SM55grAguila55grSpanish SB62gr
BBL cut to 14.5"HK 14.5" bblDifferenceBBL cut to 14.5"HK 14.5" bblDifferenceBBL cut to 14.5"HK 14.5" bblDifference
817,00837,00-20,00841,00817,0024,00829,00829,000,00
814,00824,00-10,00839,00839,000,00827,00831,00-4,00
826,00844,00-18,00844,00859,00-15,00819,00832,00-13,00
834,00834,000,00841,00832,009,00827,00844,00-17,00
853,00832,0021,00827,00857,00-30,00836,00829,007,00
Max853,00844,009,00844,00859,00-15,00836,00844,00-8,00
Min814,00824,00-10,00827,00817,0010,00819,00829,00-10,00
ES39,0020,0019,0017,0042,00-25,0017,0015,002,00
SD15,647,298,356,6217,61-10,996,076,28-0,22
AVG828,80834,20-5,40838,40840,80-2,40827,60833,00-5,40
IPSC PF149,55150,53-0,97151,29151,72-0,43168,34169,44-1,10
FPS

Hornady SM55grAguila55grSpanish SB62gr
BBL cut to 14.5"HK 14.5" bblDifferenceBBL cut to 14.5"HK 14.5" bblDifferenceBBL cut to 14.5"HK 14.5" bblDifference
2 680,452 746,06-65,622 759,192 680,4578,742 719,822 719,820,00
2 670,602 703,41-32,812 752,622 752,620,002 713,252 726,38-13,12
2 709,972 769,03-59,062 769,032 818,24-49,212 687,012 729,66-42,65
2 736,222 736,220,002 759,192 729,6629,532 713,252 769,03-55,77
2 798,562 729,6668,902 713,252 811,68-98,432 742,782 719,8222,97
Max2 798,562 769,0329,532 769,032 818,24-49,212 742,782 769,03-26,25
Min2 670,602 703,41-32,812 713,252 680,4532,812 687,012 719,82-32,81
ES127,9565,6262,3455,77137,80-82,0255,7749,216,56
SD51,3223,9327,3921,7157,78-36,0719,9020,62-0,72
AVG2 719,162 736,88-17,722 750,662 758,53-7,872 715,222 732,94-17,72
IPSC PF149,55150,53-0,97151,29151,72-0,43168,34169,44-1,10

2 comments:

  1. Very good report, Montrala! Something I and many others often wondered.

    ReplyDelete
  2. nice report.
    is this a geissele rail?

    ReplyDelete