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M16A4 build questions

1689 Views 10 Replies 5 Participants Last post by  AR15_1911
Hello, I am normally on the revolver forum but needed to post some questions over here.I am getting a Competition HBAR that I would like to build into an A4 configuration. The rifle comes without a bayo lug. Besides adding the Knights Armament rail, I'd like to change out the front sight assembly to one with a bayo lug. Has any one done this before? What challenges or problems should I expect? Any special tools needed?
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Even if you have the proper tools like a mill or a good drill press, you would have a hard time installing a new front sight base with bayonet lug on your existing barrel.

The front sight bases are generally installed on barrels womb to tomb, the two taper pin mounting holes are not exactly the same with each barrel, if you're only a couple of thousands of an inch off you can't install the second pin.

Expert AR gunsmiths could do it using a front sight base without mounting holes, but they also have a very hard time and long time getting it right, and by the time you paid for their labor it is cheaper to just buy a new barrel with the front sight base and bayonet lug and just swap your barrels.

Most AR owners just replaces their barrel assembly with either an A4 or A2 government profile barrel which is lighter and easier to carry than a HBAR heavy barrel, and they either sell their old barrel to help pay for their new barrel or just keep the old barrel for spares.

Gunsmiths in my local area usually charge around $50 or less for changing barrels (also includes installing any new rail system), or you could buy your own upper receiver action block and barrel wrench, you would also need a shop table vise to hold your upper receiver and action block.
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Wow! Thanks for the advice. Guess I will do without the bayo lug.
That's why the old SP-1 Colt's are such a good model. Bayo lug is on there already.
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Any recommendations on where I can get a mil-spec barrel and the Knight's Armament Rail?
By all means, listen to 1911 and BigG.
Can you tell us a little more about what plans you have for the gun? In a small fit of euphoria when the Clinton ban sunsetted, I grabbed an HBar with fond memories of my GI M16A1 in my head. But it didn't 'feel' right, for good reason. The HBar made it only slightly lighter than an M1 Garand.
You want a rail, so a retro gun doesn't seem to be what you want.
But for any application save shooting it off bags, you'll want a lighter barrel than the HBar.
These gents are right. I have replaced a number of AK front sight bases and a few AR ones. Fitting a new front sight base to barrels which have pre existing channels milled into them to hold the sight pins are a real crap shoot. The seldom are aligned perfectly with the rest of the rifle and rear sight, and sometimes the cant can be so bad you can't use them.

AKs haven't used front sight base pins for about 15 or so years. They now have a tool that punches and spot welds the sides of the front sight base and the gas block to the barrels. This operation leaves visible dimples on the profile sides of the two parts that go on the barrels. I thought I'd just throw this out there......a little bit of trivia for us.
My plans were to add a Knights rail (currently has a Surefire rail so I may leave it "as is") and an ACOG...that's it really. Weight is not an issue for me as I am not carrying it. Just looking for a nice distance shooter instead of using my CMP match AR.
I put together an all Colt parts A-4 about 4 years ago. I retired shortly after that and realized I could not afford the correctly calibrated ACOG scope and I had nothing else I wanted to sell! Retail is about $1300, you rarely see used ones of this model, and the M-4 ACOG is not calibrated for a 20" barrel rifle. After 100 yds. with an M-4 ACOG and a 20" bbl. accuracy starts to degrade noticeably.
The rifle shot great on a simple rest with iron sights but I intended it to be my only magnified scoped rifle. I have been short of cash since around Christmas so I sold it to my best friend in TX. What makes me feel so good about that is that it's "still in the family" and, my friend was 17 years in the USAF as a small arms instructor and armorer. He's reworked thousands of M-16 variants, yet he told me it meant something to him to have a rifle built by me. He's a sentimental sap, just like I am :)
I've seen the 20" ACOG for sale at the USMC MCX. I also have a Leupold Fire Dot VX-R that is outstanding. I may end up just using that. It works great on my M4A1 SOCOM.
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Any recommendations on where I can get a mil-spec barrel and the Knight's Armament Rail?
You could look for those on various sites that have classified ads like,,,, also auction sites like

Other places they're available are Colt dealers like and also good sources are machine gun dealers usually have M16A2 barrel replacements.

The Colt M16A4 barrels are very rare and hard to find and if you find it they want too much coin for them.

When using an A2 barrel with an A2 front sight base, you might or might not change the front sight post to a taller 0.040" post, because the A2 front sight base are 0.040" lower than the F marked A4 front sight base, you have to test it with your loads and if you can't properly zero it then change to a taller front sight post.

Don't buy the Knight's Armament M5 rail at retail prices of over $300, there are plenty of take off new or like new condition that the owner changed their mind, they usually go for $200 or less.
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