Originally Posted By PALADIN-hgwt:
How does one (properly!) ENLARGE the gas port? Is it as simple as using a drill press and the correct bit? My older Bushmaster 10.25 inch barrel is only about 0.070...
Here is my response I am pasting from another discussion that covers this pretty well:
Originally Posted By FALARAK:
Can you expound on the use of a drill bit vs a reamer for gas port sizing? i.e... the correct method to cut and/or open up a gas port? I have read about both.... but this is something I havent played with. I know when using a drill bit, you have very little control over variances in the size.... probably +-.002" if I were guessing.
I use drill bits.
Let me re-phrase, I use SHARP drill bits.
The key is that the tool must cut the metal as you are opening up the hole, not push metal.
When you are using a dull tool, the metal gets pushed ahead of the cut.
This shows up in the bore as a nasty burr and it will usually be strong as there is metal actually folded into the bore.
With a sharp drill bit, there will also be a burr pushed into the bore, but it will be very weak.
I have a brass tipped cleaning rod that I slide down the bore to knock off burrs.
They usually break off in one piece leaving very little flash.
This flashing will go away with the first shot fired, or if you are anal, you can clean the bore with a stiff brush and that usually breaks it off too.
I just shoot them out when I test fire.
Drill bit accuracy as to hole size vs marked size is more like -0.000" +0.0015", quite acceptable accuracy for the realistic accuracy needs of gas port size.
Most barrels will run just fine within +/-0.004" of the proper gas port size.
The trick to minimize burrs is to go in small steps for the last couple drill bit sizes, only cutting a couple thousandths with a nice sharp drill bit.
As for reamers, I find that most push a bigger burr ahead of the tool than a sharp drill bit that is pulling the metal away from the cut as it progresses.
It's simply a matter of cutter rake.
Drill bits have positive rake, straight reamers are neutral and spiral fluted reamers have negative rake.
Positive rake pulls the metal away from the cut, neutral and negative rake pushes it ahead of the cut.
Reamers are fine when you need very high tolerance hole diameters.
If you were drilling within 0.002" and then final reaming, that's probably fine.
The burr would be very small and weak and it will shoot out with the first bullet.
And speaking of gas port sizing - we all know Wolf ammo is underpowered. I have a Bushmaster 20" govt profile rifle, with a Vltor carbine stock. It runs great, until it gets dirty. Once it gets dry and dirty, the Wolf/low power ammo wont cycle it.... it short strokes. But it runs great on any M193/M855 ammo. If I wanted to make a rifle run more reliably on cheaper/low power ammo... is simply opening up the gas port a bit the recommended solution?
Gas port sizing to cure weak ammo is a double edged sword.
Going larger will probably cure your Wolf problems, but if you go too large, you will start getting FTE's with the full power ammo.
Going larger AND using an adjustable gas tube to tweak the action to the ammo is the best bet but some guys will tell you an adjustable gas tube is just something else to fail.
In the end, it's your choice to make.
If you do open the port, use sharp drill bits and go one wire gauge size at a time.And then there is this:
More on how gas systems work