Extra velocity mystery. 375H&H

[Float Pilot]

I have been playing around with some handloads for the CZ safari I picked up a while back in a trade, 375 H&H. I have owned a few 375s over the years, but this one actually has the potential to be a good shooter.

The former owner devalued it, because it was super in-accurate. I discovered that was because somebody had lost the bushing inserts for the thru-stock action screws. So I made a couple of bushings and she works just fine.
The only current problem is that since it has a mid-barrel support bolt, it changes zero when the barrel gets hot.
So it will put the first two shots in the same hole or touching (at 100 yards) , but then the third round is 3/4 of an inch away and any follow up shots wander out further.

CZ made the barrel lug look like a rail and the attaching thru-forestock bolt is connected in such a way that it slides on the rail. So it may benefit from glass bedding the action or maybe the entire barrel channel. As it is right now, it will ragged hole the first two shots at 100 yards with a dirty barrel, and that is just fine for a dangerous game rifle that could also take huntable game out to 200-250 yards.
I like how the action holds 5 rounds. Actually 6, but the first round sometimes hangs if you load six.

The 1.25 to 4 power Leupold 30mm scope I have on there now is nice for a dangerous game rifle, BUT I have noticed that the cross hairs are too thick and thus obscure the smaller targets I am using for hand-load development.

For some reason the CZ barrel does not like the lighter 375 bullets. The 235s and even the 260 grain Nosler accubonds shoot much slower than they should. The old 270 grain Hornadys and Speers shoot at the advertized velocities in various loading books. The 270 grain Speers at super accurate, BUT they tend to come apart when hitting bone during hunting.

The really weird part is that the old 300 grain Barnes or the 300 grain Hornady interlocks with the flat base shoot over 100 fps faster than they should.
For example:
The Hodgon book lists a load for a 300 grain sierra boat-tail using 81.5 grains of H-4350. They used a fed 215m primer and WW brass to push the Sierra BT to 2645 fps and 49,500 CUP of pressure.

I used Hornady virgin brass, a fed 215M primer and the 300 grain Hornady interlock flat base with the following charges of H-4350 ( my barrel is one inch longer at 25 inches. )

With three shot groups

81.5 grains = 2,748 fps and a 1.5 inch group. ( FIRST TWO SHOTS TOUCHING)
81.0 grains = 2,720 fps and a 1.25 inch group ( first two 3/4 inch apart)
80.5 grains = 2,713 fps and a 0,75 inch group ( first two in same hole )Nice Load. ...
80.0 grains = 2,706 fps and a 4.0 inch group ( first two 1.5 inches apart) What the heck???

Apparently this rifle is very velocity and pressure node sensitive as it it currently set up. The brass does not show and signs of super high pressure, just the slightly flat primer and slight crater edge around the primer strike dent that you would expect from a top end load with a heavy bullet. Since the Sierra boat tail used by Hodgon would stick way down into the powder, you would think that their load would have more pressure and velocity than mine.

Anyway I thought it was interesting. Considering that the lighter boat tails are 100 to 200 fps slower than they should be...

 

[macgrumpy]

When you substitute a Sierra with a Hornady it's not unusual to see the Hornady get higher speeds, that's why most people think that Hornady's reloading manual is anemic, it's not unusual for Hornady bullets to run faster.

The secret is that many of Hornady bullets are longer than bullets of the same weight built by other manufacturers. That means that when you use the same COAL there is more of the bullet seated inside the case which increases pressure and velocity.

 

[Bart B.]

Most commercial rifles change point of impact as their barrels heat up because of the way barrels fit the receiver. The receiver face is not square with the barrel tenon thread axis so when torqued in, the barrel fits tight at one place around its shoulder against the receiver. As the metal parts expand from heat, a stress line at that hard fit point makes the barrel whip more in that axis.

Have the receiver face squared up with the tenon thread axis, then put a shim between barrel and receiver to let the barrel clock into headspace and witness mark alignment.

While those barrel lugs with screws through the stock fore end into them serve some function, they are not all that good for accuracy. All three of my Win. M70's that had them shot more accurate with the screw removed and totally free floating their barrels.

I don't want any external pressure on the stock fore end changing its pressure on the barrel. Plenty of clearance between fore end and barrel is a must for best accuracy.

Regarding velocity issues, a 100 fps difference between some listed load and what your desire to duplicate it is normal. Your lots of powder and primer are different than what was used in someone else's data. And your barrel's chamber, bore and groove dimensions may be different. The neck grip on bullets may also be different. And two people shooting the same rifle and ammo will often have over 70 fps difference in muzzle velocity. We don't all hold the rifle against our bodies the same way. To say nothing of the fact that loads developed in fixed mount barreled actions will have a lot higher muzzle velocity than if it was stocked and hand held by a human.

 

[Grumulkin]

The 270 grain Speers at super accurate, BUT they tend to come apart when hitting bone during hunting.

I used the Speers on a trip to South Africa. The score:

1. One Blesbok; bang flop.
2. One Warthog; bang flop.
3. One Impala; bang flop.
4. One SCI gold medal Blue Wildebeest. That one ran about 200 yards before dieing.
5. Another Warthog that went maybe 80 yards before expiring.
6. A Zebra at about 200 yards died in maybe 25 yards by a big pool of blood.

I used no more than one bullet on any of them. I don't really care if they come apart.

 

[murf]

How do your reloads stack up to factory ammo?

murf

 

[GooseGestapo]

I think the reason the 300's are running faster is the Hornady brass. It tends to be a tad thicker than the Winchster. Heavier bullet seating deeper means slightly more pressure, hence greater velocities. With your strong action, won't be a problem.
Likely desireable as your cold temps make the pressures lower when shot is likely to count.

Regarding the lighter bullets, I found that I had to step on the powder charges a bit moe than the manuals suggested to get the accuracy I desired/expected. I was loading 225 Hornady's and 235 Speers. With RL15 and 225's in the .375Ruger, I didn't get good accuracy till I got up to 77.5gr. ; 75.0gr with the Speer 235. The Hornady is an "Interlok" bullet so holds togrther better than the Sierra's or Speers.
The new Hornady's are also the "Recoil Proof" small flat meplat with jacket to the tip construction. My 225's are the older ones.

 

[Andrew Leigh]

Float Pilot,

a coarse check in Quick Load has you at 71 000psi. I get 2 500fps with a 300gr Accubond to give 64 000psi, still over pressure.