Not to be cantankerous, but the Armalite AR15 was designed and built as a select-fire, Small Caliber High Velocity assault rifle to compete against the Winchester .224 Lightweight Military Rifle in the late 1950's. The weight of the bolt carrier isn't specific to being able to run well on automatic, as much as it is a balance against the gas system and intended charge weights with the projectiles they were working with at the time.
The weapon nomenclature "AR15" was an Armalite company designation for this military SCHV rifle, which they sold the rights off to Colt when they believed the military had no interest in it, and Fairchild, the parent company, was experiencing more losses than gains from their small arms subsidiary (Armalite). In short, the term "AR15" referred to a fully-automatic capable, select-fire assault rifle from the very start.
Notice the original selector mode locations:
![[Image: armalitear15sn1.jpg]](http://www.hunt101.com/data/500/armalitear15sn1.jpg)
Before it was type-classified by the military, Colt began production of the first series of the AR15 outside of Armalite's original tool room guns, based on the technical data from Armalite and Dutch licensed manufacture of the AR10. This is where the triangle-shaped charge handle came from (an evolution of the Dutch AR10 charge handle development). The first production AR15 was known as the Colt Model 601, and it also had "Armalite AR15" engraved on the lowers:
![[Image: DSCF5175.jpg]](http://i256.photobucket.com/albums/hh196/AFMarksman/AF%20M16/DSCF5175.jpg)
These were all select-fire assault rifles. The first semi-auto only rifles, the Colt SP-1, would not be manufactured until 1963. At some time in the SP-1 manufacturing, Colt neutered the bottom rear of the bolt carrier, so that it would be more difficult for someone to convert the civilian market rifle into a select-fire weapon.
Original AR15 (M16) Carrier on Bottom. Neutered carriers above. Not presence or lack of firing pin shroud:
![[Image: ar15_sp1_m16carriers_sm.jpg]](http://quarterbore.com/images/ar15_sp1_m16carriers_sm.jpg)
This carrier geometry and weight was an arbitrary, unnecessary part made as a "feel good" measure, in case Federal regulators might come in and claim the SP-1 was easily converted. With a Rifle-Length Gas System and 55gr ammunition, it was still a smooth-shooting rifle, where the carrier weight didn't play a big issue in weapon functionality. They also made the upper receiver without the recess for the AUTO sear, and placed a 2-piece screw assembly in the pivot pin area so that the weapon couldn't be broken down like the real AR15 could.
![[Image: 10731158_1.jpg?v=8CDFA230D122400]](http://media.liveauctiongroup.net/i/9981/10731158_1.jpg?v=8CDFA230D122400)
Fast forward to the present, 50 years after the first SP-1, and there are all sorts of carriers on the market. With the Grendel, we are talking about a cartridge that has quite a bit more case capacity than a .223 Remington, so even though the chamber pressure is lower, we use medium-slow-burning powders with the 120-140gr projectiles to get the most performance. As such, the pressure curve still has some height to it closer to the gas port than fast powders would with a 5.56 spitting 55gr, so there really is a need to balance the gas system and operating parts.
In many guns, the neutered carriers actually work fine, and can be countered with more buffer weight. In my 16" Grendel with MLGS, I use a full weight carrier and an H2 buffer (2 tungsten weights inside), and it has worked well for me. Your gas port diameter will play a big role in this equation, balancing the system out, as will projectile weight and the type and amount of powder. My carbine has cycled every 100gr NBT to 129gr SST load I have put through it 100%. The only bullet I have had issue with is the 90gr TNT, which has short-stroked on me.
As to the firing pin, all bolt types work with a standard firing pin. The AA Beowulf/Grendel bolt has extra tail length on it to accommodate a standard firing pin, since the bolt face recess is deeper. This means you have standard firing pin protrusion, and no issues with piercing. As to the question about Titanium firing pins. Here is Armalite's technical notes on them:
Quote:TECHNICAL NOTE 2: TITANIUM FIRING PINS CONSIDERED GENERALLY USELESS
PURPOSE:
The purpose of the Technical Note is to review the merits of commercially marketed firing
pins made of titanium.
FACTS:
1. Titanium firing pins are intended to reduce lock time; the time between release of the hammer and ignition of the primer. Theoretically, faster ignition of the shot allows less time for disturbance of the rifle.
2. Because titanium is lighter than the steel normally used in the production of firing pins, it has less inertia: a titanium firing pin is accelerated faster than a steel firing pin when struck by the hammer. This theoretically results in the firing pin striking the primer faster than a steel firing pin would. Lightweight firing pins or strikers have been used with bolt action rifles for many years.
3. Movement of the firing pin of the M16 type rifles is, however, only a very small part of the lock time of the rifles. Lightening the firing pin produces virtually no improvement in lock time. No engineering or experimental data has been provided which supports a change to titanium firing pins.
4. Titanium is strong, but doesn’t handle impact well. For this reason alone it is less suitable than steel for use in firing pins.
5. Titanium is lighter than steel. The steel firing pin retains a slight momentum as the bolt carrier closes. This momentum normally causes the primer to be lightly indented by the firing pin, and can cause slamfire if the primer is overly sensitive. A titanium firing pin has less momentum, causes less indent, and reduces the possibility for slamfire.
RECOMMENDATION: Demand engineering test data to support claims of accuracy improvement of any sort. We conclude that the titanium firing pin is one of many fad items separating shooters from money otherwise better spent, and recommend against them. A titanium firing pin can reduce the (already) slight possibility of slamfire. ArmaLite does not sell titanium firing pins.
Your questions are perfectly legitimate, and the only dumb question is the question not asked.
