I just finished my build : 20" stainless heavy/cmt upper/double star lower/ stag BCA and lower parts.

No binding at all when moving the bolt forward or backward, bolt slams home upon releasing bolt catch. However, when a mag is inserted regardless of which mag used, the round fails to feed and the bolt stops. This happens almost immediatley, the acutal bullet part of the round doesnt make it past the feed ramp. If I grab the charging handle and pull back and let it slam forward again, it loads the round with marks on the bullet and scratches on the casing. When the round stops moving forward, it makes a very solid sounding hit. any help greatly appreciated.

It sounds like the bullet is binding up somewhere after it has left the magazine. Try riding the charging handle forward to see how the bullet is stripped from the magazine and into your chamber. You should see an exact and hopefully consistent point at which the bullet fails to feed. You might have a burr on your feed ramp or barrel extension which is causing the tip of the bullet to get caught and sort of wedge itself into your upper or barrel extension.
The other problem that this could be the result of is a weak recoil spring. How hard is it to pull back on your charging handle? It should take a considerable amount of force to compress the recoil spring all the way back. If it's too light, FTFs are the first sign.
The final possibility is an out of spec or poorly machined bolt. Your bullet is essentially pushed by the bolt and guided by the tip. If the bolt is out of spec, it will try to alter the path of the round and can cause it to get stuck. This is a rare possibility but just something to consider.

See if any of these work. If not, try taking some pics and giving us some further details.

I tried swaping out the BCA from my olympic arms AR and it did the same thing. It makes consistant marks on the bullets everytime from the feed ramp. So it sounds like a feed ramp problem then? Im thinking maybe a trip to the range and a little break in? Maybe use a small file on the feed ramp? The buffer spring has plenty of tension, i have used it on a previous weapon with no problems.

What magazines are you using?

I have tried both my 30rd USGI and a 10rd steel i have. The barrel extension is very sharp on the edges.

this is really pissing me off now. i filed down the edges of the feed ramp a bit and the damn thing still wont feed. Has anybody had this problem before? The bullet is stoping between the grooves of the feed ramp.

The grooves of the feed ramp? Huh? My feed ramp is smooth as glass. Do you mean where your feed ramp meets your barrel extension?
Here's a long shot, but it would explain a lot. If you built this upper yourself, did you use an upper reciever with M4 feed ramps and a barrel with A2?
If you split your upper and lower, take a bullet and using its nose, feel across your feed ramps and see if there is anything you feel that may cause a problem.

Quoted: this is really pissing me off now. i filed down the edges of the feed ramp a bit and the damn thing still wont feed. Has anybody had this problem before? The bullet is stoping between the grooves of the feed ramp.

Can you get us some pictures?

Who built the upper for you?

Also, how old are your USGI mags?

Shit can the steel 10 rounder.

Bad magazines can cause those problems...

It could also be sharp edges on the barrel extension lugs digging into the brass causing the round to drag and not chamber properly...

Also check the gas tube / carrier key alignment to see if that could be slowing the bolt carrier down... Under pressure from the loaded magazine, it could be preventing the bolt/bolt carrier from stripping the cartridge from the magazine with enough force to chamber the round...

Pics would help.

Who made the Barrel? Was it a complete CMT upper, or just the upper receiver with a no-name barrel?

Quoted: ...Here's a long shot, but it would explain a lot. If you built this upper yourself, did you use an upper reciever with M4 feed ramps and a barrel with A2? If you split your upper and lower, take a bullet and using its nose, feel across your feed ramps and see if there is anything you feel that may cause a problem.

Good question. Pics would help...it would be interesting to see the feed geometry between the mag and the feed ramps. Could be an out of spec lower also.

make sure the bullets feed from the mag.  sometimes new mags need the front edge beveled or the case will hang up.  had it happen with steel 308 mags.

ok a little update....
1) mags are ok, i have tried them in my other AR
2)I changed out BCA's and both function.
3)It is a cmt upper with a no name barrel (please dont kill me)

The round were catching on sharp edges of the feed ramps, i filed them down some and now it feeds FMJ's just fine, but when using HP's it frequently jams. When it jams with the HP's they tend not to make it to the barrel extension(feed ramps) but instead hit the lower reciever just below the barrel extension. Any ideas on what to do next? It bothers me that i cant use any ammo i want, sorry i dont have pics, figures im out of the HP's now.

Quoted: ok a little update.... 1) mags are ok, i have tried them in my other AR 2)I changed out BCA's and both function. 3)It is a cmt upper with a no name barrel (please dont kill me) The round were catching on sharp edges of the feed ramps, i filed them down some and now it feeds FMJ's just fine, but when using HP's it frequently jams. When it jams with the HP's they tend not to make it to the barrel extension(feed ramps) but instead hit the lower reciever just below the barrel extension. Any ideas on what to do next? It bothers me that i cant use any ammo i want, sorry i dont have pics, figures im out of the HP's now.

I think ADCO will do M4 type feed ramps, sounds like that's what you need if you are going to shoot SP/JHP.

I guess the most obvious solution is to have a good gunsmith (read: someone you can blame if he screws up) machine out slightly larger, M4 style feed ramps.

why should i need to have m4's machined in to use that ammo? something must be wrong right? Oh and to risk sounding like a moron, the lower reciever in my Olympic AR has what looks to be feed ramp extentions? but the CMT lower doesnt.

At the risk of sounding like a die hard 1911 loyalist, the AR-15 with A2 feed ramps was never designed for JHP bullets. That's part of the reason you see M4s with, well, M4 feed ramps. Since hollowpoints have a different tip than FMJ, they run into some feeding problems since they can't be "pointed" into the chamber via the feed ramps as easily as FMJ rounds.
FYI, lowers don't have feed ramps. Feed ramps are those little notches in your upper reciever that guide the round into the barrel extension and chamber. The lower just holds the magazine, releases the hammer, and contains the recoil spring, everything else is done through the upper.

There is a reason why the M4 feed ramp was designed. You have now discovered it yourself.
Not until LE stated using JHP in AR15 carbines routinely was this really an issue, as the Military uses primarily FMJ.  However the enhancement was inmplemented in Military rifles a another improvement.

A blunt tip bullet such the typical JHP cartridge has presents a wide footprint first contacting the feed ramp, causing it to be less reliable from a feed standpoint. The extended ramps of "M4" type feed ramp enhances reliability with this type of munition by extending the ramps lower, into the receiver itself.

I am having similar problems on my KAC Stoner. I just broke everything down and cleaned it every well, it may very well be a buffer spring. It only fails to feed the very first round, after that, it's fine. 1) Magazine, or 2)Spring.

If it is a spring, how much are they and where can I get one?

Quoted: I just finished my build : 20" stainless heavy/cmt upper/double star lower/ stag BCA and lower parts. No binding at all when moving the bolt forward or backward, bolt slams home upon releasing bolt catch. However, when a mag is inserted regardless of which mag used, the round fails to feed and the bolt stops. This happens almost immediatley, the acutal bullet part of the round doesnt make it past the feed ramp. If I grab the charging handle and pull back and let it slam forward again, it loads the round with marks on the bullet and scratches on the casing. When the round stops moving forward, it makes a very solid sounding hit. any help greatly appreciated.

The mag release threads should be flush with the face of the button.  If needed, you have have to rotate the button 180* to get the thread flush.  With the mag release tight, it holds the mag tighter in the well, and prevents low nose driven rounds (contact below the feed ramps). If with the catch correctly installed you still have problem, then you can rotate the catch one more wind to see if greater mag retention solves the problem.

To add, you should be using USGI mags since some of the cheaper knock off mags do tend to have low feed jam or double feed problems. And, when you charge the weapon, pull all the way back on the charging handle, and let it run home under
it's own power. Walking the charging handle forward will cause a jam of sorts everytime.


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Quoted: I am having similar problems on my KAC Stoner. I just broke everything down and cleaned it every well, it may very well be a buffer spring. It only fails to feed the very first round, after that, it's fine. 1) Magazine, or 2)Spring. If it is a spring, how much are they and where can I get one?

Did you check the end of spring winding to see if there is a end bur that may be scrapping down the inside of the receiver extension and causing the problem?

If you need a new spring, there are many suppliers like Bushmaster that sells new recoil springs for around $10.

Quoted: At the risk of sounding like a die hard 1911 loyalist, the AR-15 with A2 feed ramps was never designed for JHP bullets. That's part of the reason you see M4s with, well, M4 feed ramps. Since hollowpoints have a different tip than FMJ, they run into some feeding problems since they can't be "pointed" into the chamber via the feed ramps as easily as FMJ rounds. FYI, lowers don't have feed ramps. Feed ramps are those little notches in your upper reciever that guide the round into the barrel extension and chamber. The lower just holds the magazine, releases the hammer, and contains the recoil spring, everything else is done through the upper.

oops i meant the upper. Ya that makes sense that the non-m4 style upper would have feed probs. It seems like that is my problem. It runs FMJ's flawlessly and I was hoping to use PSP's for ground hogs, coyotes, ect. I think i wll take it to a gunsmith for the feed ramps, thanks so much for all your help.