View Full Version : A barrels chamber type and it's part in barrel accuracy.
How much does chamber type (between 5.56 NATO vs Wylde,USMC SAM-R,etc.) play or gain you in an AR15's barrels inherent accuracy, if all the other barrel characteristics are exactly the same when shooting only the heavier 68-77gr match type ammo ? Does a Wylde,USMC SAM-R, or Noveske Match Chamber gain you maybe 1/2 MOA accuracy over the standard 5.56 NATO chamber ?
The only inherent difference between 5.56 NATO and the 223 Wylde chamber is the leade in front of the chamber, which is the area where the bullet "jumps" to the lands of the rifling. Theoretically, the most precise chamber will have the shortest leade with the least amount of bullet "jump". With that, a SAAMI 223 Rem. chamber will theoretically be the most precise out of them all. 5.56 NATO has the longer leade to safely run the higher pressure 5.56 ammo. 223 Wylde has a leade somewhere between the two to safely run 5.56 NATO and 223 Rem, but still retain the shorter/accurate leade associated with the 223 Rem chamber.
Now that's not to say a 5.56 NATO chamber will not be accurate with match ammo. A lot of them are, but if I was going for the most accurate rifle possible, I would go 223 Wylde since it will safely fire 223 Rem AND 5.56 NATO, but still be very accurate. My 18" match grade Wylde chambered rifle will put 5 shots in a quarter with my 77gr handloads. Also, rifle twist can make or break you; a 1:7 or 1:8 twist is needed with the long and heavy 77gr bullets.
The following is a picture is a cutaway of the two chambers for reference. Between the orange and yellow line is the leade. Notice how the 223 Rem leade is short and pronounced while the 5.56 NATO leade is long and gradual. In theory, this "jump" is what hinders accuracy.
http://www.strelec.si/vanilla2/uploads/FileUpload/1996_011f5de2043a62f014cd7d31829b3e6d.jpg
From my understanding, the 223 Wylde,USMC SAM-R, Noveske Match Chamber, etc. are all very similar to the 223 Wylde chamber.
5.56 NATO has the longer leade to safely run the higher pressure 5.56 ammo.
The 5.56 isn't loaded to higher pressures. It's loaded to make full pressure in the 5.56 chamber with it's longer leade. 223 Remington ammo is loaded to make full pressure with a shorter leade.
When 5.56 ammo is fired in a 223 SAAMI chamber with it's shorter leade, pressures rise above normal.
When 223 ammo is fired in a 5.56 chamber with it's longer leade, the ammo doesn't reach it's full pressure and some velocity is lost.
Where accuracy may be lost with a longer leade is if the bullet enters the rifling cocked. Entering the rifling cocked means it will exit with a wobble. If the longer leade is cut so the bullet always enters the rifling straight, the rifle will shoot more accurately.
I suspect no manufacture is cutting their 223 chambers to true SAAMI specs because of the prevalence of 5.56 ammo. If If Brand R cuts their 223 chambers to true SAAMI specs and has to issue a warning their rifles are not safe for use with 5.56 ammo, they're going to lose sales to Brand W who cut their leades just a little longer to be safe for use with all ammo
Failure2Stop
12-17-13, 07:10
How much does chamber type (between 5.56 NATO vs Wylde,USMC SAM-R,etc.) play or gain you in an AR15's barrels inherent accuracy, if all the other barrel characteristics are exactly the same when shooting only the heavier 68-77gr match type ammo ? Does a Wylde,USMC SAM-R, or Noveske Match Chamber give you 1/2 MOA or more accuracy over the standard 5.56 NATO chamber ?
There is a reason that so many folks go through the trouble of creating a new chamber.
I do not believe that the chamber alone is going to improve precision by 1/2 MOA, but probably up to 1/4 MOA.
The problem is that this is very hard to test without a whole lot of barrels to compare side by side along with a whole lot of ammo and time.
It is also important to note that there are two other very important characteristics of a chamber/throat design.
1. Lead Angle - this is the area of the throat where the lands have been cut to creat the "funnel" that introduces the bullet into the bore.
SAAMI spec .223 Rem has a VERY steep lead angle of 3 deg 10min, where a 5.56 NATO has a lead angle of 1 deg 20 min.
The 223 Rem will generate pressure issues with 556 NATO ammo due to the short throat AND the lead angle.
2. The other aspect is free bore diameter. This is the area before the actual throat where the lead angle starts. It is a straight cylindrical section.
SAAMI .223 Rem will typically have a free bore dia of .2242" and a NATO chamber will show .2265-.2270" Clearly, one can see the ability of the bullet to get "loose" in the freebore.
Bear in mind, the 223 Rem was designed back in '64 and was done so with bullet of the very light variety ~ hence the short throat.
The 556 NATO although derived from the 223, was "loosened" up to allow functioning with dirty / out of spec ammo.
Jack's comment above is spot on - there is a plethora of 223 / 556 reamers that have been "re-engineered". I own a number of them. I have JGS grind mine and they are all optimized for specific uses.
One is a 556 Trophy - specifically dimensioned for shooting mag length 69/77gr match ammo for across the course (200-600yds) matches.
Another is the 556 Target - specific to optimal length match ammo utilizing 80gr Sierra's.
Still another is my 556 LR - set up for the 90gr VLD's for 1000yd use.
All share the tightest free bore that I can safely / reliably run ~ .2240-.2242". The first two use 1 deg 30 min leads while the LR uses a 1 deg 10 min lead (matched to the VLD design secant type ogive.)
The Wylde chamber is a great chamber - it is a do it all type and one can expect very good accuracy with a huge range of bullets. Bill Wylde was a pioneer in the development of the 223 as a viable competition / long range cartridge. His chamber generally runs a little loose in the neck - even more so than a 556 NATO. Numbers wise the neck dia at the mouth are .254 for a 223 Rem & the 556 NATO. The Wylde runs .2558". The neck dia at the shoulder is .255 for both the Rem & NATO, while Wylde's runs .2568"
The Wylde has a tight free bore just like the Remy.
One very big difference is the free bore lengths - the Remy run .025", the NATO is .0566 and Wyldes is .0619 - longer that the 556 NATO - and one of the reasons it handles pressure as well as 80gr projectiles.
There are many other minor differences as well - specifically in case body diameters. Unfortunately, some companies still chamber in 223 Rem. I have Rem 700 SPS in 223 and it has a SAAMI spec 223 Rem chamber - it is SHORT!! I only shoot 50-52gr pills out of it and it is very sensitive to seating depth.
Hope this helps.
5.56 NATO has the longer leade to safely run the higher pressure 5.56 ammo
The 5.56 isn't loaded to higher pressures. It's loaded to make full pressure in the 5.56 chamber with it's longer leade. 223 Remington ammo is loaded to make full pressure with a shorter leade.
When 5.56 ammo is fired in a 223 SAAMI chamber with it's shorter leade, pressures rise above normal.
When 223 ammo is fired in a 5.56 chamber with it's longer leade, the ammo doesn't reach it's full pressure and some velocity is lost.
I completely agree with you as I was referring to the 5.56 ammo being loaded to a higher pressure (62,000 psi) than the 223 Rem (55,000 psi) as if they were shot through the same chamber/barrel. As it appears we all agree on, the pressure generally depends on the leade of the chamber.
Keep in mind that NATO pressure and SAAMI pressure are measured differently and cannot be directly compared.
The longer leade is a way to increase case capacity as it takes very little pressure to push the bullet out of the case. When case capacity is increased, there will be a drop in pressure and velocity using the same charge of powder. This means more powder will be needed to obtain the same velocity but usually results in a drop in pressure which in turn, will allow a further increase in powder charge to bring pressure back up to normal and a possible increase in velocity.
Reloaders have long known every rifle barrel is a law unto itself and when chambering a barrel, trade-offs are made in an attempt to get the performance that is looked for, whether it's ultimate velocity, pure accuracy, reliability above all or some balance between the three. Even so, a reloaders tailors the round to match the chamber of the rifle as possible to get either best velocity, best accuracy, best reliability or the best balance of the three
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