I do want to mention we did see some flattening of primers with the MKZ in his ARP barrel, but I didn't see that in my DD barrel, so obviously the lead is having an effect there. But we didn't see any flattening of primers in the DHA SST on either, brass looked immaculate for both despite the 50 FPS advantage of the ARP (one of the reasons I recommended it to him was this form and some of the info I had read before joining, since at that time DD stopped selling their 6.8 barrels for builders).
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Lead Length Vs. Press on 110gr vs. 120gr
1 - 12 of 12 Posts
There is more going on than you realize. For one thing, the ARP chamber is considerably more sophisticated than a normal SPCII. It was pretty much designed to get as much as possible out of the factory ammo available that was available roughly around 2008 and uses several tricks to do it. For example, most SPCII chambers have a parallel-walled section of freebore of about 0.278" diameter. After that they taper at 1.5° per side to and through the rifling. This means that the bullet does not encounter the rifling until some distance beyond the end of the parallel-walled freebore. Although Constructor has not broadcast the exact details of his 6.8 ARP chamber to the world, we know that his tolerances are different and generally tighter than for typical for SPC and SPC II. At one point he mentioned a tighter freebore and some areas of his chamber design where he maintains a very slight taper instead of parallel walls. This moves the point of contact of the bullet and the lands farther back, reduces blowby and can change how the bullet engages the lands (better alignment). His barrels also have a higher groove-to-land ratio than most on the market. All of these tend to increase the velocity, but there still is a bit of unpredictability due to other factors.
As to why one particular bullet/powder/case/primer combination shows a bigger advantage than another, that comes down to extremely complicated dynamics that still cannot be predicted. It involves properties of the powder, bullet, harmonics, etc. It is known that very small changes can have a significant impact on muzzle velocity. Sometimes the stars line up and velocity goes up more than expected, other times, not.
You can get a taste of these confounding results by looking at the bullet velocity data generated as a barrel is slowly cut back. Most people would expect a consistent change in velocity or at least a discernible, consistent pattern, but that is not seen in reality. Slight changes in barrel dynamics produces a surprising variation in velocity. This can be seen in numerous tests posted on the Rifleshooter.com website. The author starts with a long barrel chambered in a particular cartridge and cuts it back one or two inches at a time. If you look at the results, the exit velocity decreases inconsistently and occasionally even increases after a section is removed. The author typically tests several types of ammo and the inch by inch velocity change differs with each type. There is a lot going on that we still don't fully understand.
rifleshooter.com
As to why one particular bullet/powder/case/primer combination shows a bigger advantage than another, that comes down to extremely complicated dynamics that still cannot be predicted. It involves properties of the powder, bullet, harmonics, etc. It is known that very small changes can have a significant impact on muzzle velocity. Sometimes the stars line up and velocity goes up more than expected, other times, not.
You can get a taste of these confounding results by looking at the bullet velocity data generated as a barrel is slowly cut back. Most people would expect a consistent change in velocity or at least a discernible, consistent pattern, but that is not seen in reality. Slight changes in barrel dynamics produces a surprising variation in velocity. This can be seen in numerous tests posted on the Rifleshooter.com website. The author starts with a long barrel chambered in a particular cartridge and cuts it back one or two inches at a time. If you look at the results, the exit velocity decreases inconsistently and occasionally even increases after a section is removed. The author typically tests several types of ammo and the inch by inch velocity change differs with each type. There is a lot going on that we still don't fully understand.
223 Remington/ 5.56mm NATO barrel length and velocity: 26 inches to 6 inches
Have you ever wondered how much barrel length effects muzzle velocity on your rifle? Well I did, so I cut up rifles chambered in 223 Remington, 308 Winchester, 7 mm Remington Magnum and 300 Winche…
rifleshooter.com
Thanks, I actually read that article before, the pressure curves are exponential, kind of like a capacitor charging in electronics. You get a lot more change near the bottom end than at the top end. I think those kinds of properties are where the 14.5" came from, they found the point where you start to really drop off with further reduction, but lengths longer than that has significantly diminishing returns for that cartridge. That's why I've always stuck with 14-16" barrels for my 5.56 stuff.There is more going on than you realize. For one thing, the ARP chamber is considerably more sophisticated than a normal SPCII. It was pretty much designed to get as much as possible out of the factory ammo available that was available roughly around 2008 and uses several tricks to do it. For example, most SPCII chambers have a parallel-walled section of freebore of about 0.278" diameter. After that they taper at 1.5° per side to and through the rifling. This means that the bullet does not encounter the rifling until some distance beyond the end of the parallel-walled freebore. Although Constructor has not broadcast the exact details of his 6.8 ARP chamber to the world, we know that his tolerances are different and generally tighter than for typical for SPC and SPC II. At one point he mentioned a tighter freebore and some areas of his chamber design where he maintains a very slight taper instead of parallel walls. This moves the point of contact of the bullet and the lands farther back, reduces blowby and can change how the bullet engages the lands (better alignment). His barrels also have a higher groove-to-land ratio than most on the market. All of these tend to increase the velocity, but there still is a bit of unpredictability due to other factors.
As to why one particular bullet/powder/case/primer combination shows a bigger advantage than another, that comes down to extremely complicated dynamics that still cannot be predicted. It involves properties of the powder, bullet, harmonics, etc. It is known that very small changes can have a significant impact on muzzle velocity. Sometimes the stars line up and velocity goes up more than expected, other times, not.
You can get a taste of these confounding results by looking at the bullet velocity data generated as a barrel is slowly cut back. Most people would expect a consistent change in velocity or at least a discernible, consistent pattern, but that is not seen in reality. Slight changes in barrel dynamics produces a surprising variation in velocity. This can be seen in numerous tests posted on the Rifleshooter.com website. The author starts with a long barrel chambered in a particular cartridge and cuts it back one or two inches at a time. If you look at the results, the exit velocity decreases inconsistently and occasionally even increases after a section is removed. The author typically tests several types of ammo and the inch by inch velocity change differs with each type. There is a lot going on that we still don't fully understand.
223 Remington/ 5.56mm NATO barrel length and velocity: 26 inches to 6 inches
Have you ever wondered how much barrel length effects muzzle velocity on your rifle? Well I did, so I cut up rifles chambered in 223 Remington, 308 Winchester, 7 mm Remington Magnum and 300 Winche…
1 - 12 of 12 Posts
