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Old 08-29-2012, 06:07   #1
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cutting down a 20"HBAR to 16"

I`m interested in cutting down and recrowning my 20"HBAR to 16"....Am I going to create any problems with the gas system or anything else...Jim
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Old 08-29-2012, 06:46   #2
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Sounds interesting, you would have a 16" barrel with rifle length gas tube. I wish I could answer the question for you, best I can say is: I don't see why it would cause a problem. It would look really cool to have the barrel shortened and free float it with rifle length hand guard so you only had a couple inches of barrel sticking out of the hand guard.
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Old 08-29-2012, 07:56   #3
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It will work, but you may need to to a bit of tuning to make it function reliably.
The issue is dwell time. With rifle-length gas on a 16" barrel, the bullet has barely passed the gas port before it exits the muzzle, resulting in (maybe) not getting enough gas volume and pressure to reliably cycle the action.
OTOH, your rifle might work just fine without any tuning needed. The only way to know is to do it.

Options:

1) Only cut the barrel to 17 or 18 inches. This hardly seems worth it to me, but might be exactly right for you. The little bit of extra barrel length increases dwell time.
2) Lighten up your buffer by removing some of the weights. This mostly seems to help the full-autos.
3) Probably the most effective (and correct) way is to open up the gas port. You might need to go as large as 0.125 inches. It's not hard to do, just a bit scary the first time you run a drill bit into your barrel. Make sure you have a cleaning rod in there, for when the bit punches thru into the bore: you don't want to hit the other side.
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Old 08-29-2012, 13:27   #4
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Originally Posted by PigBat View Post
It will work, but you may need to to a bit of tuning to make it function reliably.
The issue is dwell time. With rifle-length gas on a 16" barrel, the bullet has barely passed the gas port before it exits the muzzle, resulting in (maybe) not getting enough gas volume and pressure to reliably cycle the action.
OTOH, your rifle might work just fine without any tuning needed. The only way to know is to do it.

Options:

1) Only cut the barrel to 17 or 18 inches. This hardly seems worth it to me, but might be exactly right for you. The little bit of extra barrel length increases dwell time.
2) Lighten up your buffer by removing some of the weights. This mostly seems to help the full-autos.
3) Probably the most effective (and correct) way is to open up the gas port. You might need to go as large as 0.125 inches. It's not hard to do, just a bit scary the first time you run a drill bit into your barrel. Make sure you have a cleaning rod in there, for when the bit punches thru into the bore: you don't want to hit the other side.

Thanks for the reply...I was thinking the dwell time might be the killer...someone said that there`s a company that makes a 17" with
a rifle length gas system and it works fine and they guarantee it but they
will not guarantee a 16"....So it looks like the 16" is a no go...so you`re
right...I`ll have to figure if it`s going to be worth it to do it....Jim
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Old 08-29-2012, 14:02   #5
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Ideally:
A 20 inch barrel would have rifle-length gas.
A 16 inch barrel would have mid-length gas.
A 14.5 inch barrel would have carbine-length gas.

Most of the Dissipators I've seen run mid or even carbine-length gas under the rifle-length handguards, but there are some that run rifle-length gas. I don't know offhand how big their gas ports are, but I'm betting they've been opened, if for no other reason than to improve reliability across the spectrum of possible ammo.

You can make a "non-ideal" combination work, and opening the gas port is easy and quick, so don't dismiss the idea completely, unless you don't want to have to tweak this rifle.
In that case, you'll be better served by not mucking about with your HBAR's barrel: just pull it, sell it and install a 16" mid-gas barrel in its place.
A barrel swap is easier and quicker than a cut, crown, drill out gas port, etc.
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Old 08-30-2012, 20:54   #6
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Originally Posted by PigBat View Post
Ideally:
A 20 inch barrel would have rifle-length gas.
A 16 inch barrel would have mid-length gas.
A 14.5 inch barrel would have carbine-length gas.

Most of the Dissipators I've seen run mid or even carbine-length gas under the rifle-length handguards, but there are some that run rifle-length gas. I don't know offhand how big their gas ports are, but I'm betting they've been opened, if for no other reason than to improve reliability across the spectrum of possible ammo.

You can make a "non-ideal" combination work, and opening the gas port is easy and quick, so don't dismiss the idea completely, unless you don't want to have to tweak this rifle.
In that case, you'll be better served by not mucking about with your HBAR's barrel: just pull it, sell it and install a 16" mid-gas barrel in its place.
A barrel swap is easier and quicker than a cut, crown, drill out gas port, etc.

Thanks....I haven`t given up yet but a new barrel is starting to look like a far simpler choice.....but even better is to purchase a complete upper...I didn`t want to spend the money but it really isn`t a enormous cost....We`ll see
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Old 09-01-2012, 14:49   #7
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jimjc,

In the end, you'd probably be better to just get a second upper half. Besides, this would give you options when you wanted to flex the a rifle into a different role.

Of course, it's simple enough to swap to a different barrel, provided you have the tools and the know-how. In fact, if you choose to go this route, you may want to consider a "disippator" barrel...16" with rifle length handguards. DPMS, Del-Ton and other manufacturers make them.

Take note, however, that there are two kinds of "dissipator" barrels...those that have a rifle-length gas system...and, those that utilize a mid-length or a carbine-length (it varies by manufacturer) gas system that resides underneath the handguards. The latter utilizes a handguard retainer that is affixed to the pinned FSB.

I prefer the rifle-length version myself. In fact, I used a 16" DPMS dissipator barrel on a A2 build project I did last winter. I left this one without any tactical bells and whistles; just a straight-forward AR. But, I really like the look and feel of the rifle.

This is what mine looks like:
DPMS Tactical 16

FYI...I used RRA receivers for this build, along with the DPMS dissipator barrel and A2 buttstock, as well as DPMS internals.

Let us know how it goes.
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