Brass fouling in SIG 550 rifles

[Bollocks]

Gents,

Hoping you might be able to help I have the following question.

I've been shooting brass jacketed 224's for about 1/2 a year now in 4 AR15 barrels, a Steyr AUG and a SIG 550 rifle. In the AR's and Steyr AUG the fouling is normal. Comparable to copper jacket fouling rates and nothing out of the ordinary.

However, in the SIG (semi-auto military style rifle, 20 inch barrel) I see this weird fouling. I use a Hawkeye borescope for inspection and for the first half down the barrel fouling looks normal, just like in the other rifles. But then a few centimeter before the gasport thick layers/streaks of brass start to form. Looks caked. They last for a a couple of centimeters and the fouling suddenly stops again (still before the gasport). You can also see the fouling without the borescope and it looks like the barrel is sowly being choked at the point of the fouling.

Weird thing is that a friend of mine who has 2 SIG 550's experiences the same fouling at the same position in both his rifle. When the barrels are clean that part of the barrels does not look rough. We cannot find any apperent reason for this behaviour.

So I'm hoping someone here can help to find the cause. One thing I can think of is because these barrels are cold hammered and maybe that part is tighter. But I'm guessing here and have not measured this yet. Also the factory could not provide any plausible answers.

Thanks from Holland,
BolloX

[JonB_in_Glencoe]

by your discription of hte appearance,
sounds like molten brass.
I am no expert, But I'd guess that may be the hottest part of the barrel.
Jon

[buck1]

I have heard of this type of fouling. The article I read sugested outers foul out rod to get rid of it.

[runfiverun]

it's gas cutting. where the gas in the bbl is going in the hole and is escaping around the bullet.
just like gas cutting on a cast boolit.

[Bollocks]

Thanks for your suggestions Gentlemen.

[sagacious]

Weird thing is that a friend of mine who has 2 SIG 550's experiences the same fouling at the same position in both his rifle.

Thanks from Holland,
BolloX
Are you both using the same powder for these loads? Are you using ball or extruded powder?

[Bollocks]

Hi Sagacious,

I used Vitha N135 (stick) and Hodgdon H335 (ball). Made no difference.

My friend tried prb pcl 506 (ball), vihtavuori N130-N133-N135 and N140. Heavy and mild loads. All with the same results. Also sorting 22 cases by brand did not make a difference.

The Swiss use this rifle as their std service rifle and they had similair issues using nickel-tombac jackets. They switched to tombac jackets in the late 90's which cured the issue. Sadly I have hard time retrieving more info from that side.

Our 3 rifles were made 8-10 years ago and a SIG 550 with a barrel manufactured 4 years ago did not show this problem. The production of the barrels was moved from SIG Neuhausen to Sauer some years ago because SIG Neuhausen had 'issues' with the barrels. But it is unclear to me what these 'issues' were.

Thanks,
BolloX

[Calamity Jake]

it's gas cutting. where the gas in the bbl is going in the hole and is escaping around the bullet.
just like gas cutting on a cast boolit.


Sence he said it happens before the gas port I would say it is a restriction, not gas cutting.
I don't know what the outside contour of the 550 barrel looks like but if it has a turned down area at that location that was installed after the barel was rifled then it is causing the restriction, or maybe the gasblock is to tight.

[Bollocks]

Hi Calamity Jake,

Thanks. That is also the direction I'm thinking in. These barrels were cold-hammered so it could very well be a tighter spot. But I have not measured this yet.

Also I find it weird that with copper (J4) jackets this fouling does not appear.

Thx,
BolloX

[dale2242]

I can`t help with the solution to your problem, but i have shot 1000s of junkyard bullets in my Ruger Hawkeye 223 with no excess fouling. I have fired up to 500-600 shots before cleaning ...dale

[Bollocks]

@Dale - the SIG rifles are the only ones where we see this excessive fouling. The other rifles like AR15's and Steyr AUG are fine.

This is the barrel profile

http://www.whq-forum.de/cms/uploads/pics/sig550-02.jpg