Seating depth dilemma

[el matador]

I'm working up a load for my 300 Win with 168 grain Sierra TMKs. Trying to get a lower recoil, plinking load for paper and steel out to 600.

To touch the lands on my rifle I need 3.570" OAL, which has me seating the bullet only 0.160" into the neck. These are short bullets! The neck on a 300 WM is only about .260" long, so if I seat to that depth I'm .100" off the lands.

My first round of load development (.120" off lands) gave me several of those groups with 2 shots touching and one flier. That makes me want to mess with seating depth, but how much longer can I go?

 

[Corey Schwanz]

A general rule a lot of people like is a minimum of half the caliber's worth of bearing surface actually in the neck. For a Win Mag (or any 30 caliber), that'd be .154" of the bearing surface in the neck. You just need to make sure you have sufficient tension to hold the bullet and not let it slide it or out.

 

[el matador]

Thanks Corey, that's the info I was looking for! I talked to Josh at Cooper rifles and he recommended SAAMI standard 3.340" oal with the regular match kings. The tipped bullets are around .145-.150 longer so I'm going to explore an oal around 3.490. Good to know I can try even closer to the lands if that doesn't work out. Hoping to get into the .2s with this setup.

 

[Bart B]

That general rule of some caliber, or fraction thereof, for a bullet's heel to be seated that far back from the case mouth has been debunked so many times. I've no idea where that myth came from. Nor has anyone ever made a plausible argument for it based on the physics of what happens when a round's fired. It reminds me of chapter 13 in the book "How To Make Things Do What You Want Them To Do" titled "Electronic Slot Machines." It says to use two fingers touching the screen first in the upper left-hand corner, then the lower right-hand one, then the upper right-hand corner and finally the lower left-hand corner to have the greatest odds of making the next play a winner. I've seen many people in casino's doing that, but some of them insist that order's wrong and a different one is correct.

Best proof it's a myth is people getting best accuracy for any given caliber don't use one of those standards such believers specify. Another's Sierra Bullets use of bullets up to 240 grains in the .300 Win Mag test barrel used for all 30 caliber bullet 190 grains and heavier getting 1/2 MOA at worst test groups in their 200 yard range. I've shot bullets from 155 to 220 grains from several 30 caliber cartridges with excellent accuracy with all sorts of seating depths relative to bullet diameter.

I suggest seating the bullets so the .300 Win Mag's COL's 1/16th inch shorter than magazine length for good functioning. I did that with a custom barreled .300 Win. Mag with a SAAMI spec chamber with 165 to 220 grain bullets. A wide range of bullet heel location back from the case mouth didn't seem to matter as bullet jump to the lands varied quite a bit and accuracy stayed at the same good level across all; no worse than 1 MOA at ranges 500 yards and further.

For single round loading, seat them out to touch the lands which helps center them in the rifling. If they're held with only 1/8th inch of the neck length, so be it. That won't be a pressure problem with reduced loads.

Regarding your test groups.... It's my opinion there are no such thing as fliers. All of those shots striking far away from the aiming point went there because the shooting system put them there. All the variables added up in that direction. The ammo, rifle and the thing that aims the rifle comprise the shooting system. It's the thing that holds the rifle that has the widest range of variables. Shoot at least 10 shots per test group. Kudge accuracy by the largest group for a given load fired. A 3-shot group have about a 1 in 5 odds of representing where all shots fired with that load will shoot inside of; it only tells you where those three went.

 

[Bart B]

Typical .300 Win Mag Throat Erosion Rate

With a given bullet jump distance to the rifling that's established as best starting out, note the bullet's jump distance to the rifling will increase .001" for every 10 to 15 rounds fired. All rifle throats advance with barrel wear, but I don't think one needs to chase the lands as they move forward. There's a few hundredths inch spread in bullet jump distance where accuracy changes very little, if at all.

 

[el matador]

Re: Typical .300 Win Mag Throat Erosion Rate

Well it's sounding more and more like I have nothing to worry about by seating closer to the lands. I can touch the lands with this bullet and still have .160" of bearing surface inside the neck. They will easily fit in the magazine at that length.

This gun will put the 208 Amax into 3/8 moa so I know it's just a matter of finding the right load.

 

[Rich Coyle]

Recently I discovered my new rifle seems to prefer a jump of about .170". I started jammed and kept going away from the lands in .030" increments.

 

[deadidarren]

Recently I discovered my new rifle seems to prefer a jump of about .170". I started jammed and kept going away from the lands in .030" increments.

but did you increase the powder charge to compensate for the velocity drop

when you seated off the lands .

 

[rcoody]

but did you increase the powder charge to compensate for the velocity drop

when you seated off the lands .

Be aware that, as you load your cartridge progressively shorter, putting the bullet deeper into the case, you will be reducing the effective case capacity dramatically. With smaller cases, such as the .223 Rem and 6mmBR, moving from .010″ into the lands to .080″ and .120″ off the lands can CAUSE a dramatic pressure rise. So, a load .010″ into the lands that may be safe can be WAY OVERPRESSURE with the bullet seated .120″ off the lands (i.e. .130″ deeper in the case, the difference between .010″ in and .120″ out).

 

[deadidarren]

Be aware that, as you load your cartridge progressively shorter, putting the bullet deeper into the case, you will be reducing the effective case capacity dramatically. With smaller cases, such as the .223 Rem and 6mmBR, moving from .010″ into the lands to .080″ and .120″ off the lands can CAUSE a dramatic pressure rise. So, a load .010″ into the lands that may be safe can be WAY OVERPRESSURE with the bullet seated .120″ off the lands (i.e. .130″ deeper in the case, the difference between .010″ in and .120″ out).

yes but not when he was .020" of the lands on his fist .030 move

 

[Rich Coyle]

but did you increase the powder charge to compensate for the velocity drop

when you seated off the lands .

I didn't change anything except the seating depth. I doubt it is over pressure since it holds 109 grains of water to the top of the case. A .264 holds about 80 grains to the top of the case and runs a 125 grain bullet at 3,200 feet per second. My load is running a LRX127 at 3,250 feet per second.