Colt 10.3 FSP Question

**Bowser** · 05-23-23, 14:57

What size would you use?

**Dutch110** · 05-23-23, 15:53

Originally Posted by Bowser

What size would you use?

Me? I am going to run it as it shipped, see how it goes, and adjust from there. I'm not experienced with this set up at all so wouldn't pretend to give anyone advice. There is a ton of different opinions on the topic it seems. I did a quick perusal of published specs for port size on the 10.3 - 10.5 FSP barrels

Aero / BA - .0785
SOLGW - .073
LMT - .071 (I think I am going from memory here)
Colt - .070
TDP - .070
Daniel Defense - .070 now but from threads on here they originally shipped with .083 and were a tad on the gassy and bouncy side as a result.

I'm also going to be running a can. Which is also going to play into this. Which is why I am going to run it and go from there and adjust as needed.

**Uncas47** · 05-23-23, 16:55

Seriously tempted.

**Dutch110** · 05-23-23, 17:04

Originally Posted by Uncas47

Seriously tempted.

Mine shipped today and is showing up Friday. If you can hang on I'll let you know how it looks.

**Clint** · 05-23-23, 17:40

The Crane spec of 070 is just large enough to reliably run 556 ammo un-suppressed.

That 070 spec size will also function acceptably with most suppressors, but is quite a bit over driven and gassy.

it is hardly the right size for dedicated suppressed use.

**Dutch110** · 05-23-23, 18:00

Originally Posted by Clint

The Crane spec of 070 is just large enough to reliably run 556 ammo un-suppressed.

That 070 spec size will also function acceptably with most suppressors, but is quite a bit over driven and gassy.

it is hardly the right size for dedicated suppressed use.

Thanks Clint. Appreciate it. In a perfect world what do you think is the correct port size for that application?

**Clint** · 05-23-23, 18:27

The correct gas port size depends on many factors, including the back pressure of the particular supressor model, but most fall just under 060.

That .010" drop from 070 to 060 is a BIG deal and represents a 26% reduction in flow area.

Originally Posted by Dutch110

Thanks Clint. Appreciate it. In a perfect world what do you think is the correct port size for that application?

**Dutch110** · 07-05-23, 10:28

Finally got this all put together and to the range over the weekend to shoot it with and without the can (Otter Creek Polonium)

1. When I got the barrel in I swabbed the chamber and it came out clean. Ran a few patches down the bore as well and that came out clean too. Visual inspection saw no fouling or port erosion. I don't think this barrel had been shot much, if at all.
2. Got to the range and shot it without the can to get it on paper. Indoor range (can is still in jail so I was limited to indoors at 25 yards for my conjugal visit so keep that in mind)
3. First 60 - 100 rounds groups tightened up as expected with a new(ish) barrel.
4. Running an A5 with Sprinco green and an A5H2 buffer. Recoil impulse was good and ejection pattern was right around 3:00. It was a lot softer than I thought it would be to be honest.
5. Put the can on and ran another 100 rounds through it. Groups continued to get smaller. I was actually surprised by the lack of gas to the face. Using a Radian CH so no gas diversion. This same can on other rifles is a bit gassy so I was expecting this to be the same if not more so.

Conclusion? 200 rounds or so is by no means a thorough evaluation but initial impressions are that my sample size of one barrel from this lot is pretty darn good. Maybe I got lucky and got one that sat on the armorers shelf? With the can and an RDS and after breaking it in a bit it was consistently .5 to .75 at 25. So probably a 2 to 3 inch gun at 100? Which for a 10.5 milsurp barrel I am fine with. On and was shooting 55 grain Igman .223.