Benchmark8208, neck tension, col

[AUSSIETRIGGER]

G'day from Australia

I'm very knew reloading and I have always come here for information. But now I have a few questions that I'm sure can be answered by the experienced reloaders.

First I'm using Hornady brass due to not being able to get my hands on anything else. Powder is benchmark8208. Primers are remington 7 1/2s. Projectiles are blitzking 39grs. The rifle is a tikka t3 varmint stainless steel, vortex viper hs 4-16x50.

The first batch I made used 26grs and grouped ok but blew soot down the necks. The group printed about 1/2 an inch high at 100yards. I re-neck sized my cases(they were on the looser side) and tried another group same charge. But this group printed bang on zero at 100yards. Why would this be? I would have thought more neck tension would increase velocity.

The other thing, I started seating to factory col, but I'm now seating to magazine length (about .080 difference) and this hasn't really made any difference to group sizes. But does it have any other advantages?

Sorry for the long post.

[Darkker]

Increased neck tension won't matter to a tangible extent on velocity, the brass simply can't hold that tightly on the bullet. What it affects is start Pressure and keeping the bullet aligned to the Bore, prior to engaging the rifling. Remember that barrels are a harmonic system, so any change to timing or harmonic waves will change the output; as what you know as groups.

Seating deeper will always (to a certain point) lower pressures and thus, velocity. The more jump, the more gas can bypass the bullet, and lower Pressure. There was a bunch of work done around the 50's IIRC, by i believe Dr. Brownell showing this. Until you get VERY deeply seated, the Pressure continues to drop. The potential other effect is if you have a large jump to the lands, is the bullet can get off-kilter before it engages the lands; no bueno for accuracy.