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2/21/2020 11:35:28 PM
Posted: 1/1/2015 8:45:25 PM EST
[Last Edit: 1/2/2015 12:50:03 AM EST by Blain]
A few years back, I purchased ARF member Swingset's custom Bushmaster 18" A2 upper. Can see his post here

Picture of the gun in his original configuration.

He had a Bushmaster AR15A2 HBAR that he sent to ADCO, had them cut it down to 18" and reprofile it under the handguard from HBAR to medium profile. I don't have a caliper, but the barrel looks to be around .700 diameter.

He claimed it was very accurate, but I never got around to shooting it. It had an ugly fake YHM phantom style muzzle break that was 2" long, almost defeating the purpose of cutting the barrel down in the first place! I had planned to replace it out with a standard A2 flash hider before I shot it, and it mostly just sat in storage. Last year I got curious about 18" setups, and had one custom built. It is one of my favorite rifles now, but it also rekindled my interested in Swingset's upper. I just recently got a FH for it, swapped it out, and stuck it on an A2 lower.
FYI, I timed the A2 flash hider so that the center port was at 1:30, which is how Colonel Lutz aka Coldblue, who designed it, said it was supposed to be timed if being shot by a right handed shooter. Allows the muzzle to recoil with the shooter for better control.

Because ADCO doesn't open up the gas port when they cut down a barrel, I knew I had to use a custom BCG to ensure positive reliability. For the record, the military uses a .104 size port for 18" barrels, Bushmaster's 20" rifle gas port is likely around .093 - .096. Much smaller than needed to reliably cycle 5.56 under all conditions, nevermind if I wanted to use low powered .223 or steel case!

Luckily, I had the perfect BCG saved for just such an occasion. It is a colt half circle BCG, fully shrowded, that was custom coated in nickel teflon by Jojo's gunworks (also bought off the EE). I had learned with my earlier project that having a lighter BCG, esp one with a specialty coating that produced a lower coefficent of friction, was essential in such a modified gas system to ensure reliable cycling. My first 18" project would occasstionally short stroke in the extreme cold with tula until I installed a chrome plated semi BCG. The lighter weight of the semi carrier along with the decreased coefficent of friction, helped the gas system cycle more positively. The half circle BCG is even lighter than the semi, with an even slicker coating, so I figured it would be perfect to use for this smaller gas port.

The complete rifle, after my modifications, with a web sling and no magazine weighs 7 lbs. and 9 ounces (taking that ugly muzzle break off actually saved a full ounce and a half of weight as well as an inch in length). This is just a hair lighter than a standard 20" A2 with a GI profile barrel (those weigh in at around 7 lbs. 10 / 11 ounces on average in the same configuration. The rifle balances really nicely! Here she is in all her glory.

I took her out to the range today and shot her for the first time. I used new production Federal Xm193 for all shots fired. It was around 30 degrees F out. All shots were fired at 100 yards from the prone position utilizing a hasty sling hold and 6 o'clock sight picture with iron sights.

The first thing I noticed was just how softly this rifle recoiled. The 20" .093 gas port with a 18" barrel feels like a .22lr! Not that my first custom 18" upper recoiled hard with its .102 size gas port, but it was slightly snappier and more noticeable than a standard 20" rifle gas. I couldn't believe how lightly this rifle recoiled! All of the brass ejected at around 4 / 4:30.

I rapid fired a quick string of shots just to check where I was hitting so I could adjust for zero. the almost non existent recoil really assists with the follow up shots!

This is what my first group looked like.

I was very happy with the group considering how I was shooting and the ammo I was using. I quickly calculated my adjustments. 10 clicks left windage, 4 clicks up front sight post.

I fired another string, a little slower this time.

This is what happened!

I think I'm in love!

The rifle was dead on, exactly where I wanted it to be, and the group was better than I expected! I can't believe Swingset let this thing go!

Not to be outdone, I took out my original 18" A2 project, custom built by Braceman. This build incorporates an FN CHF GI profile 18" barrel. The gas port is opened up to .102, and the rifle has a semi auto fully chrome plated BCG. The rifle in this configuration with websling and no magazine weighs exactly 7 lbs. 6 ounces. Slightly lighter due to the .650 barrel profile underneath the handguards. This rifle also has an extended A2 flash hider so that it can still accommodate a bayonet for SHTF compliance (swingset's upper has a neutered lug, so didn't bother on his).

Picture of this bad boy!

I fired a couple strings with her to see how she compared. Utilized the same firing conditions.

Not too shabby, though I'm not sure what accounted for that grouping of shots on the right there. You can definitely notice the difference in recoil between the two guns. Not that this gun recoils a lot, but it is just noticeable compared to the almost lack of recoil from the first rifle with the unmodified port!

Later on when it gets really cold (single digits or lower), I am going to take both of them out again and do a cycling reliability comparison utilizing low powered .223 and steel cased ammo.

All in all, it was a great start to the year. The more I experiment with 18" barreled A2s, the more fond of them I become.
Link Posted: 1/2/2015 12:28:32 AM EST
Nice! Glad it's working out for ya...it's a good upper and a nice package.
Link Posted: 1/2/2015 4:29:08 PM EST

For what it's worth, I'm pretty sure my Colt A2 20"er is right at 7 lbs 8 ounces. Or 7.5. I can double check. But I have the neutered bolt and no bayo lug. Could be the difference.
Link Posted: 1/2/2015 6:05:19 PM EST
Well well. This thread has me thinking about making a change to one of my rifles. I love
shooting iron sights over optics. I also have an extra A2 upper and a set of Colt
A2 handguards that need a new home. Really liking the idea of an 18" A2.

Link Posted: 1/2/2015 6:32:30 PM EST
Nice rifle.
Link Posted: 1/3/2015 2:36:20 AM EST
Discussion ForumsJump to Quoted PostQuote History
Originally Posted By JJREA:

For what it's worth, I'm pretty sure my Colt A2 20"er is right at 7 lbs 8 ounces. Or 7.5. I can double check. But I have the neutered bolt and no bayo lug. Could be the difference.
View Quote

Remember, my listed weights include a web sling which adds 2 to 3 ounces. ;)
Link Posted: 1/3/2015 3:10:58 AM EST
beautiful rifle man.
Link Posted: 1/3/2015 10:06:31 AM EST
Real nice 18" A2's Blain, thanks for sharing.
Link Posted: 1/18/2015 12:04:27 PM EST
[Last Edit: 1/18/2015 3:42:35 PM EST by Blain]
Cold Weather Range Report

First, a historical perspective...

Anytime a non standard custom configuration AR is built, with any possibility of being put to serious use, it is vital to test for reliability and functioning of the platform during adverse conditions it may be required to be used in.

With 18" rifled gas barrels, there can be concern about proper functioning during cold or fouled conditions with ammunition due to there being less port pressure than the standard 20" barrel length that is standard with that gas system. The reason why many choose to enlarge the gas port on the 18" rifled gas barrels, to compensate for this difference.

When I first assembled my custom A2 18" upper, I had no experience with the 18" platform. I wanted it to be as reliable as a 20" gun, and I assumed I would have to use a port at least .104 in size, same as the military's MK12. As a result, when I had the upper assembled, I begun with a .1015 size gas port as I figured I would have to have at least that size to function properly.

After it was first assembled, it was test fired in temperature of the single digits unlubed with tula steel cased ammo. It functioned flawlessly and ejected at around 3 o'clock. It held the bolt open after the last shot.

Once receiving the upper, I test fired it with a lot of very dirty tula (had a fine coating of what looked like powder on it) unlubed with a FA fail zero BCG. I had some short strokes and failures to feed (the NiB would get dirty and carbon would stick to it). I even had some stuck cases in the chamber, one which had to be mortared out! After switching carriers to a melonited version, the failures to feed stopped...for a while! I eventually still had issues however.

Later on I got a semi auto chrome BCG that I retested with (unfortunately, the new tula I used in the replacement tests wasn't as dirty and covered in grime), it was then that my functioning issues went away, and it had cycled flawlessly since.

The Test!

I wanted to do my own cold weather testing of the system. Especially now that I had an 18" rifled gas upper with an alternative gas port size to contrast it to. I wanted to compare the differences between them in cold weather functioning. I wanted it to be as cold as possible. Unfortunately, the coldest day of the year so far, which was in the negative digits last week, was on a work day and I couldn't do any shooting.

However, yesterday morning came close enough to make it worthwhile!

It was around 5 degrees F when I started to shoot. I used a brownell's tan follower USGI mag to ensure that any difficulties would not be magazine related. I was testing 4 ammo types; PMC bronze, Tula 55 grain fmj, Fed XM193, and Fed Fusion 62gr. The ammunition had been left outside all night so they were the actualy ambient temperature. I did not bring outside room temperature ammo to conduct the test as that would defeat the purpose.

The PMC and Tula are especially known for causing functioning problems as the PMC is very anemic even for a .223 load (over 100 fps velocity lower than comparable fmj 55 grain .223 loads). The Tula is steel cased and has an unusual and inconsistent pressure curve that can often cause cycling issues.

These rounds were known to give problems cycling mil spec ARs under standard conditions, how would they perform on these non standard variants during such cold climate conditions?

Just to give you an idea of how cold it was (and why electronics can not be relied upon in arctic conditions), it was so cold out that my cell phone would power itself off when turned on. It was too cold for it to remain functional!!

Rifle A
At first I test fired some 10 round strings of Tula and PMC through my custom 18" A2. I am going to herein refer to this gun as "Rifle A". It is the A2 I had custom assembeled with the FN CHF 18" barrel, full chrome plated semi BCG, and enlarged .1015 gas port. It ran flawlessly with around 4 o'clock brass ejection and held the bolt open after each string.

Rifle B

I then did the same with swingset's regular gas port sized 18" A2 upper (Rifle B). I had put a colt half circle nickel teflon coated BCG in this upper to try to compensate for the 20" sized .093 gas port. Surprisingly....it functioned perfectly with both the Tula and PMC, holding the bolt open and ejecting the ammo at around 4:30. This rifle recoiled much less and a lot smoother than Rifle A, yet still retained perfect functioning!

Just as a point of reference, the weight (confirmed by digital scale) of a full auto BCG is 11.6 ounces. A semi is 11.4 ounces. The colt half circle is 11 ounces even.

I fired a few more strings of each ammo through both rifles, then decided I would test out a Surefire 60 that I had kept continually loaded for 2 years since I first got it.

The surefire was loaded with 60 rounds of federal fusion, it was fully loaded and stuck away for two years. I inserted it into Rifle A and began firing. After around 35 rounds or so, I begun having feeding problems. The gun was not fully chambering. After a few instances of this, I pulled the mag out. The follower was actually stuck 2" down the magazine! After some taps it correct itself. I then loaded the surefire into Rifle B and fired the rest of the 25 or so rounds without incident.

I then took out the XM193 for the heck of it, and I'm glad I did. Since all of the "problem ammo" cycled fine, I never would have expected to have any difficulty firing the full power stuff! Ironically, that is exactly what happened.

I started with Rifle A, and though the recoil and cycling was noticeably harsher than with the .223 loads with "perfect" 3 o'clock ejection, it did NOT hold open the bolt at the end of the last round! I fired some more strings, It cycled perfectly, but would not hold open the bolt!

I then tried it in Rifle B. Again, Rifle B fired with much less recoil than Rifle A. It also functioned perfectly, and it DID hold the bolt open after the last round! This is not what I expected! If anything, the smaller gas port sized rifle should be the one having difficulty locking the bolt back....but that's not what happened!

I broke open each rifle and swapped BCGs. Rifle A fired the Fed XM193 with the Colt half circle nickel teflon BCG. The difference in BCG mass was definitely noticeable as Rifle A's ejection pattern changed from 3 o'clock to 2 o'clock! It then locked the bold back after each round!

I then test fired the Fed XM193 in Rifle B which now had the chrome semi auto BCG in it. I expected problems, but though the ejection pattern moved closer to 5 o'clock, it still cycled perfectly and locked open the bolt after the last round!!

At a loss, I swapped both BCGs back to their original rifles. I fired some more strings of Fed XM193 out of Rifle A with the chrome BCG back inside it. Now it was all of a sudden locking the bolt back after the last round of each string...


I am somewhat as a loss here, as the results were totally different than what I was expecting. The good news is that both rifles feed and function perfectly under adverse cold conditions. I do find it odd though that the larger gassed rifle had problems locking the bolt back, where as the "undersized" 20" gas port rifle has no cycling or bolt hold open problems whatsoever. According to swingset, he put around 4k rounds through the upper before I bought it, so the gas port might have eroded and enlarged a little bit, but not to any degree where it would even mach Rifle A's .1015 port.

Does anyone have any theories as to what might be going on here?

Below is just a fun picture. Even though I was function testing, I abhor just blasting into the dirt, so I set up a little target on the 20 meter line just so I had something to shoot at, though I was by no means shooting for any kind of accuracy. Looks like the windages are pretty much dead on though!

Link Posted: 1/18/2015 6:28:27 PM EST
Do you suppose that the full power M193 caused rifle A to pick up so much bolt velocity that it bounced foreward off the rear of the receiver extension before the follower had a chance to engage the bolt catch assembly?
Link Posted: 1/18/2015 7:42:02 PM EST
Honestly, I don't know what it could be. If I didn't have my prior experience with the short stroking I'd think it was over gassed. If I had a full auto carrier, I would have tried it to see what difference it would have made. Honestly, I did not expect something like this to happen. I expected something like this might happen to the .093 sized gas port rifle, but it didn't. Making it all the more confusing.
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