When the first generation Ruger SR-556's  with the quad rail came out, I bought one as my first AR.  Love the rifle, it's accurate but a bit on the heavy side.  After I got the AR bug I added a lightweight rifle to my collection, so I decided to make the SR-556 a prairie dog gun.  The changes to the SR-556 were minimal, a Ruger Elite 452 trigger, BCM Mod 1 compensator, A2 fixed stock with rifle buffer and rifle length spring, Magpul grip, and a quality scope.  I run the gas system on setting 2 as setting 1 seemed to short stroke the action.  The end result is the SR-556 is an accurate shooter.

So here's my problem.  When I initially charge the rifle (I'm not riding the bolt) or when I'm shooting a string, the bolt doesn't always strip the next round from the magazine.  I can't tell if the bolt isn't moving with enough force to strip the round from the magazine or if the rounds are possibly hanging up on the feed ramps.  This happens sporadically with different magazines and ammo.  I run Magpul magazines and quality brass cased ammo through the rifle (found out real quick the SR does not like steel cased ammo).  The SR's bolt, when charging the rifle, seems to move slow and sluggish, it isn't as snappy as my other AR's in stripping an initial round from the magazine.  So here are my questions, is this slow bolt/sporadic failure to strip the round from the magazine caused by my changing from the adjustable carbine stock to the fixed A2 stock, rifle buffer and rifle length spring?  Do my feed ramps need to be polished?  Or have I missed something else that could be causing this problem?

Thanks for the help.

Change it back to the way you bought it ...if it works you know the  changes to the riffle length buffer tube ,spring and buffer is your problem. If it still doing the same thing ...not stripping a round when you charge it then send it back to Ruger.

I agree with the above post. Its probably something to do with your mods. So put it back to its carbine configuration. If it works you know where the problem lies. If not a trip back to Ruger is called for.

Thanks for the replies.  I ordered a new rifle length spring, thought I would give that a try first.  If that doesn't fix the glitch I'll shift the Ruger back to factory form with the adjustable stock.

When you installed the A2 stock and rifle buffer you increased the buffer weight substantially. From a short googling session the stock buffer should be a typical carbine buffer at 2.95 ounces. A standard rifle buffer of the sort typically included with an A2 stock weighs 5.2 ounces. My guess is Ruger sized the gas port on the SR-556 to run the lighter carbine buffer.

Swap back to your stock components or better yet slap the upper on a lower with a carbine buffer and take it to the range. If this fixes it you can get a spacer to put in the rifle extension so you can run a carbine buffer and spring. Click here for an example of a spacer from Brownells. I believe someone makes a delrin spacer that should be lighter than aluminum.

For what it's worth I run an H3 buffer(same weight as a rifle buffer) with my AA mid length upper. It's fine with 5.56 but can be a bit sluggish with TulAmmo 223.

I have two SR 556 model rifles.  I built both lowers and just purchased the uppers back when Ruger did that kind of thing.
One is 6.8 SPC and I am too lazy to dig it out and check the buffer that I went with when I built the lower but it is probably a regular carbine weight buffer.
The 5.56 C has an anti tilt H3 buffer which weighs 5.5 ounces.  I have not had any issue with either rifle.  Is the anti tilt worth the extra $$?  maybe / maybe not.  I don't shoot either one of them enough to do a real test.
Both rifles are very accurate for regular production guns.  A lot of people talk shit about them, especially the Ruger 6.8 because it is SAAMI chamber but I think they are better guns than they get credit for being.  Target below is factory SSA 115gr using a 1.25-4x VX-R @ 4x.

Attached File

Thanks for the suggestion on the spacer.  I haven't had a chance to get out and shoot since the rifle length spring came in.  If the new spring doesn't take care of the problem I'll swap out lowers and see what happens.  If that takes care of it, looks like a spacer may be the fix I need.

Just to test, do this:

Remove the rifle bidder and place on a table.
Place a stack of quarters on the table with the carbine buffer on top.
Continue to increase the stack of quarters until the height of them plus the carbine buffer equals that of the rifle buffer.

Place the quarters into the rifle length buffer tube, install the carbine spring and buffer.
Try a few shots at the range to ensure operation.

Then order either a alloy or delrin spacer.
Be sure you remove the quarters.

Finally got to go shooting and sort out my problem with the Ruger.  
When I pulled the old buffer spring out and compared it to the new rifle length spring I ordered, the problem was obvious.  The old spring was 1.5 inches shorter than the new one.  I bought used parts to convert my rifle to the fixed A2 stock and took it for granted all the parts were OK.  The used spring that came with the parts was longer than the spring in my carbines so I thought it was good to go.  Switched out the buffer springs and she ran like a champ.  No glitches with a mix of different magazines and ammo.
Thanks for the suggestions that were offered.