Ok i'm gonna keep this as simple as i can.
Last weekend i made it to the range with my M4gery (colt) and another Colt AR with a 20 inchs barrel (sporter), 100 rounds of norinco ammo, 200 rounds of remington UMC and 50 rounds of Magtech ss109 green tips.
The M4 was running on an M16 bolt group (CDNN), while the sporter was using it's own non full circle AR bolt group.
I started breaking in my M4 (the sporter had been used previously) so i fired 100 rounds of norinco without a hicup, then, 200 rounds of UMC again with no malfunctions whatsoever in a moderate rate of semi auto fire. After firing those 300 rounds, it was time to try that new ss109 magtech ammo which i had so i insert a mag, load a round into the chamber and squeeze the trigger only to find out that the spent case had not been ejected (it had been extracted but not ejected and the bolt had caught it against either the upper receiver or the barrel extension, parallel to the ground and rear side foward, meaning the neck of the case was now touching the face of the bolt)...so i'm like WTF?!?!, clear the jam, insert a new round into the chamber, stand up (i was shooting from a bench and thought i could have been resting the gun on the mag) and fire a round, no joy, the same malfunction again.  At this point i'm thinking, is the carbine short stroking or is this just weak ammo?  (i had fired 300 rounds of other brands with no problems) so i take the sporter out, feed a magazine into it, pull back from the charging handle and insert one of these freaking rounds into the chamber, squeeze and bang, bang, bang...so at this point i'm almost sure, the carbine was short stroking, but just for the hell of it, i decided i'd swap bolt-carriers (i did make sure everything was locking up ok), giving now the m4 the AR bolt group (and carrier) and giving the sporter the m16 bolt (yeah, the block on that sporter's been filed down, which isn't an issue down here, read my location) only to find out that the malfunction had moved with the bolt/carrier and now the M4 was running fine, in other words, now the sporter is -jamming- (exact same malfunction).  Both bolt-carriers are colt. So now i'm thinking "this piece of shit ammo isn't hot enough to make an M16 bolt cycle?!?! it appears so (the m16 carrier as most of us know is heavier) what do you guys think?
Both Gas keys are tight one is steaked and the other isn't, but they are BOTH tight.

HUNTER.

Anyone? i'm simply asking for an opinion here, come on guys.

HUNTER.

Do bolt swaps in the carriers to narrow it down to if it's just the bolt, or just the carrier that is the problem.  
A tight key doesn't always mean a key that isn't leaking!

Also, it could be as simple as one carrier hammer binding, and the other not. Look for a bright line/indent behind the hook on the disco that would be caused on such a bind.

Added: When you do the bolt/carrier swaps, remember to single round load and check for bolt lock back on the catch on the empty chamber (short stroke test).  

Quoted: Really no such thing as "weak ammo" with the AR. That's a shotgun phenomenon, but even then it not "weak" just designed for a specific purpose and won't function well in a semi-auto shotgun designed for a conflicting purpose (light bird loads in a gun designed for heavy slug or defense loads).  ARs are gas-operated, so as long as there's a good bullet seal in the barrel, the gun should cycle with ANY 5.56 or .223 ammo out there. Gas port issues are something of an anachronism nowadays.  back when barrels were being cut down from 20", the gas ports had to be opened accordingly, which led to trouble.  Nowadays factory barrels are easily had in a variety of lengths and the gas ports are dead-on correct for the barrel length.  As long as it's a factory barrel (by just about ANY factory), poor cycling can be blamed on other things.

Yes, No, Maybe,

In regards to the barrels and gas system, it all boils down to this.

The round is fired, the bullet passes the gas port, then exits the barrel.  From the time that the bullet passes the gas port, then leaves the barrel, the gas system has to pressurize, and when the bullet leaves the barrel, the barrel pressure has to drop enough to allow the chamber fire formed case to release off the chamber wall (lose enough pressure tension from the chamber wall that the extractor can pull the spent case out).

If we are talking a true spec USGI chamber like those produced from Colt or FN (read slightly wider chamber side wall dimensions), the yes to an extent that over cycling is not produced.

Now on the other hand, since Semi AR-15 barrel/rifles are sold on there merits of being target type accurate, the chambers are reamed on the tighter side (side wall dimensions for less blow-by during case fire forming) and this means that the normal allowed variances in system pressurization to barrel bleed off at case pull (pressure two point timing) can be greatly diminished. The tighter the chamber, and the shorter the barrel/ port from muzzle, the smaller the working margin. (here we get the no answer).

Now since we are talking Semi auto rifles, with tighter than USGI chamber, we have to throw in chambering headspace size (distance and throat angle).  As of now there are three types of chambers for the rifle.  There is the 223, the 5.56 Wylder, and the 5.56 Nato chambers.  In regards to these chambers, depending on what ammo is shot threw these chambers, the working barrel pressure of the ammo can vary from the low side (223 ammo in a Nato chamber) to the over pressure  (5.56 Nato ammo fired in the 223 chamber).  Since again, the action cycle all boils down to enough pressure to cycle the system, and the barrel pressures lowered enough to allow the spent case to be pulled/release from the chamber walls (really timing of the bolt pull when the case pressure has drop low enough to be pulled), this can be a deciding factor as well.

Bottom line is the government has set the standard on combat reliability specifications for chamber size (head space/ chamber side wall dimensions), needed gas port sizes, ammo pressure, and acceptable group size (all threw contractor standard specifications/ R&D in upgrade programs).   But here is the rub, in order for the commercial rifles producers to “One Up” each other, their focus has been more on lowering the groups sizes that their barrels will produce over the standard USGI groups, and each other (other barrel commercial producers). So when doing this tighter chamber upgrade (?), the standard working margin of the standard USGI M-16 systems have been reduced (sometimes considerable like in the 10.5" commercial barrels).

To sum it up in regards to barrel choice since now some of you will be reeling in regards to new barrel choices, If you’re looking for a combat reliable barrel/system, then the chamber/gas port dimensions that Colt and FN produce for the government will typical produce groups of around 3 MOA with standard Nato ammo.  On the other hand, if your looking for a commercial barrel that will produce tighter groups, then these barrels will be more ammo sensitive.

Quoted: Quoted: Really no such thing as "weak ammo" with the AR. That's a shotgun phenomenon, but even then it not "weak" just designed for a specific purpose and won't function well in a semi-auto shotgun designed for a conflicting purpose (light bird loads in a gun designed for heavy slug or defense loads).  ARs are gas-operated, so as long as there's a good bullet seal in the barrel, the gun should cycle with ANY 5.56 or .223 ammo out there. Gas port issues are something of an anachronism nowadays.  back when barrels were being cut down from 20", the gas ports had to be opened accordingly, which led to trouble.  Nowadays factory barrels are easily had in a variety of lengths and the gas ports are dead-on correct for the barrel length.  As long as it's a factory barrel (by just about ANY factory), poor cycling can be blamed on other things. Yes, No, Maybe, In regards to the barrels and gas system, it all boils down to this. The round is fired, the bullet passes the gas port, then exits the barrel.  From the time that the bullet passes the gas port, then leaves the barrel, the gas system has to pressurize, and when the bullet leaves the barrel, the barrel pressure has to drop enough to allow the chamber fire formed case to release off the chamber wall (lose enough pressure tension from the chamber wall that the extractor can pull the spent case out). If we are talking a true spec USGI chamber like those produced from Colt or FN (read slightly wider chamber side wall dimensions), the yes to an extent that over cycling is not produced. Now on the other hand, since Semi AR-15 barrel/rifles are sold on there merits of being target type accurate, the chambers are reamed on the tighter side (side wall dimensions for less blow-by during case fire forming) and this means that the normal allowed variances in system pressurization to barrel bleed off at case pull (pressure two point timing) can be greatly diminished. The tighter the chamber, and the shorter the barrel/ port from muzzle, the smaller the working margin. (here we get the no answer). Now since we are talking Semi auto rifles, with tighter than USGI chamber, we have to throw in chambering headspace size (distance and throat angle).  As of now there are three types of chambers for the rifle.  There is the 223, the 5.56 Wylder, and the 5.56 Nato chambers.  In regards to these chambers, depending on what ammo is shot threw these chambers, the working barrel pressure of the ammo can vary from the low side (223 ammo in a Nato chamber) to the over pressure  (5.56 Nato ammo fired in the 223 chamber).  Since again, the action cycle all boils down to enough pressure to cycle the system, and the barrel pressures lowered enough to allow the spent case to be pulled/release from the chamber walls (really timing of the bolt pull when the case pressure has drop low enough to be pulled), this can be a deciding factor as well. Bottom line is the government has set the standard on combat reliability specifications for chamber size (head space/ chamber side wall dimensions), needed gas port sizes, ammo pressure, and acceptable group size (all threw contractor standard specifications/ R&D in upgrade programs).   But here is the rub, in order for the commercial rifles producers to “One Up” each other, their focus has been more on lowering the groups sizes that their barrels will produce over the standard USGI groups, and each other (other barrel commercial producers). So when doing this tighter chamber upgrade (?), the standard working margin of the standard USGI M-16 systems have been reduced (sometimes considerable like in the 10.5" commercial barrels). To sum it up in regards to barrel choice since now some of you will be reeling in regards to new barrel choices, If you’re looking for a combat reliable barrel/system, then the chamber/gas port dimensions that Colt and FN produce for the government will typical produce groups of around 3 MOA with standard Nato ammo.  On the other hand, if your looking for a commercial barrel that will produce tighter groups, then these barrels will be more ammo sensitive.

Dano, i'm running an COLT M4A1 SOCOM barrel (the one that's 4oz. heavier compared to the govt profile M4 barrel) 14.5 1/7 barrel twist, so my guess is it's chamberis milspec.  On a side note this barrel (and bolt-carrier)  was cycling every other brand of ammo used (so it could or could not be a weak extractor spring, but since it is an interesting and cheap to fix theory, i will give that a try ---thanks homeinvader---)...as far as the groups go, Dano, let me tell you brother, this barrel is acuratte as hell.  On the first trip to the range, i was doing half an inch groups at 50 meters using IRONS! i really was not expecting that kind of accuracy, some of the bullet holes were very very close to the next one.

HUNTER.

Quoted: the working barrel pressure of the ammo can vary from the low side (223 ammo in a Nato chamber)

IIRQ, magtech is loaded on the low pressure side from the start, and this alone could be the deciding factor on carrier mass/weight dictating if the rifle cycles or not.

Quoted: Quoted: the working barrel pressure of the ammo can vary from the low side (223 ammo in a Nato chamber) IIRQ, magtech is loaded on the low pressure side from the start, and this alone could be the deciding factor on carrier mass/weight dictating if the rifle cycles or not.

Could be...the same m16 carrier-bolt is cycling  just fine (100%) --using that ammo that does not work in the CAR-- on the rifle and never (0%)  in the carbine now, i checked everything i could check, gas tube alignment, gas leaks, changed the extractor spring for a colt blue insert one etc etc, now get this, when i fire the CAR using the AR carrier it feeds perfectly and locks back after last round is fired...now, when i try to run that m16 bolt-carrier in it, using that same ammo (the one i think is weak) the damn thing wont even pass the short stroke test (let me again remind you that the rifle passes it every time using the m16 carrier). I'll break that bolt gruop in using some other ammo ( the carbine is functioning 100% reliably with every other brand of ammo). IMHO the carbine short stroking, it's either that or the ammo is weak (correction the ammo is not MAGTECH but CBC) which means "compañia brasilera de Cartuchos" they have along time worked together sometimes, so even so they ARE diferent companies quality is pretty much the same.
just "eliminating variables here".


HUNTER.