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Some thoughts on porting a 16" barrel...


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I have a FM9 with a 16" barrel and a threaded comp.

I know the comp isn't doing much good that far from the chamber.

So I thought about shortening the barrel to 10" and pin and weld on a Brekke 6" comp.

 

But I would like to try a little cheaper route.

Partly because the prices over here are about twice what they are in the US.

But also because it's fun to do it yourself...

 

So I was thinking about porting the barrel at about 6" from the muzzle.

But I'm a little concerned that I will loose too much bullet speed...

 

So I'm thinking about drilling out the last 4" of rifling in the barrel, with a 10mm drill, so it won't loose as much speed.

 

The OD of the barrel is only 16mm,

With gives a thickness of about 3.5mm.

so I think several small holes would be better than a few large holes at directing the gases straight up.

 

Has anyone here tried anything similar?

Any thoughts?

 

 

 

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If you port the barrel......as a rule, yes, you want to get rid of the rifling after the port. If you port at say 14"......you probably would not need to drilling out the rifling......but at 6" absolutely. I personally would not go that far back......in the testing we did, 8 to 12 is the area you want to look at, with 10" being the sweet spot.

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6 hours ago, TRUBL said:

If you port the barrel......as a rule, yes, you want to get rid of the rifling after the port. If you port at say 14"......you probably would not need to drilling out the rifling......but at 6" absolutely. I personally would not go that far back......in the testing we did, 8 to 12 is the area you want to look at, with 10" being the sweet spot.

Thanks a lot for the input!

I really value it.

My original plan was actually to build a AR9 with taccom components (I emailed you about it last summer), but since I couldn't get my hands on that here in Sweden, I went with a FM instead.

Too bad about the US export restrictions :(

 

Do you mean that the sweetspot is centering the ports at 10", or placing the ports after 10"? (like with a 10" barrel)

 

And do you have any advice on what diameter I need to use when drilling out the rifling?

I was thinking about drilling with a 10mm drill first (maybe 4-5" deep).

But is there a risk that the bullet touches the barrel because of the recoil?

Should I drill the last section (maybe 3" deep) with a larger diameter (12mm?)...?

 

The reason for not going 12mm all the way, is I'm thinking about drilling a few holes after the rifling as well (more like a comp than a porting)

And I think those holes would be more effective with more goods thickness...? 

 

And do you have any input on the diameter for the port holes?

I'm thinking about drilling (or actually milling to avoid burrs) 5mm holes...

 

Is there a risk that I end up with a barrel that overcompensates and actually dips on recoil, if I drill to many ports and comp holes...?

 

I should mention that I'm no fan of handloading, so I want it to work well and keep a PF of at least 125 with standard pistol ammo.

Preferably 115gr, but heavier bullets is ok as long as it's easy obtainable standard ammo.

 

I know I'm probably over analyzing everything...😳

 

 

 

 

Edited by grottulf
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I've realized that drilling out the rifling is not a realistic option, due to the surface hardening of a barrel...

At least not as deep as I wanted to go.

 

I spoke with a friend who is truly awesome with both a mill and a lathe.

He works professionally with them and does custom work on a daily basis.

(I have both mill and lathe myself, but I really suck at using them 😳)

 

As we discussed it, we came to the conclusion that it would be better to cut the barrel at 10", and shrink fit a tube over the barrel. 

 

I've also abandoned the idea of porting where there is rifling, due to the risk of getting burrs.

 

But I could make the extension with a smaller inner diameter at the beginning, to get better effect from the comp holes, and with a larger ID at the muzzle to avoid unnecessary weight and minimize the risk of contact with the bullet.

 

But now when I examine the pictures of the Taccom super feed 10/16 barrel, I realize that's probably exactly how you made that...?

 

So I realize I'm trying to reinvent the wheel here. 😳

 

It sucks that I can't get a hold of those barrels on this side of the pond ☹ 

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On 2/20/2021 at 3:47 PM, grottulf said:

I've realized that drilling out the rifling is not a realistic option, due to the surface hardening of a barrel...

At least not as deep as I wanted to go.

 

I spoke with a friend who is truly awesome with both a mill and a lathe.

He works professionally with them and does custom work on a daily basis.

(I have both mill and lathe myself, but I really suck at using them 😳)

 

As we discussed it, we came to the conclusion that it would be better to cut the barrel at 10", and shrink fit a tube over the barrel. 

 

I've also abandoned the idea of porting where there is rifling, due to the risk of getting burrs.

 

But I could make the extension with a smaller inner diameter at the beginning, to get better effect from the comp holes, and with a larger ID at the muzzle to avoid unnecessary weight and minimize the risk of contact with the bullet.

 

But now when I examine the pictures of the Taccom super feed 10/16 barrel, I realize that's probably exactly how you made that...?

 

So I realize I'm trying to reinvent the wheel here. 😳

 

It sucks that I can't get a hold of those barrels on this side of the pond ☹ 

Do you have to have a 16" barrel where you live? Could you machine yourself a comp like this? 

https://brekkecustom.com/index.php/product/pcc-compxtension-6/

 

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6 hours ago, Chillywig said:

Do you have to have a 16" barrel where you live? Could you machine yourself a comp like this? 

https://brekkecustom.com/index.php/product/pcc-compxtension-6/

 

It is possible to get a permit for a shorter barrel in Sweden, if I compete in a sport that allows shorter barrels.

In IPSC you're allowed to compete with a shorter barrel, but for some reason they don't give approvals for  shorter barrels.

(You must always have an approval from a sanctioned shooting association, when seeking permit for a gun. If it's not for hunting then it's easier, but then you are much more restricted in what you can get.)

So I'd like to keep it at 16" for legal reasons.

How you measure the length isn't   really defined in the law here though.

 I interpret it like you guys, and think that pinning and welding a threaded comp would be ok.

 

As I said in the first post, I was comptemplating buying a Brekke comp.

But I'm leaning towards building one myself, with help from a friend.

 

Instead of threading it, I'm thinking of making a shrink fit between the barrel and comp, and press it on.

 

I'd also like to make it in aluminum to keep the weight down.

But I don't think that would work with aluminum, since that expands more than steel when the temperature rises.

 

Maybe I'll just bite the (9mm) bullet, and get a Brekke comp after all...🙄

 

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