Effects of long freebore

C

Colorado Redneck

New member
I have a CZ in 222 Rem. The barrel length is 22 inches. To determin OAL of handloads, the method of using a spent case that is modified by cutting 3 length wise slots into the case mouth and seating whatever bullet to be loaded, then inserting the case into the chamber and closing the bolt. Then to double check, placing the bullet into the barrel throat and tapping it in lightly, and using a wooden dowel to determine the distance from the bolt face to the bullet tip (more complicate than this, but no need to explain more).

The freebore seems to be fairly long. To seat 0.03 off the lands with Nosler 40 BT, the COL is .050 longer than specified in the SAAMI spec. So all of this is well and good. The issue is, velocities are significantly less than the load data indicates for everything I have put through this gun. 100 fps less is perfectly acceptable, but 250-300 fps seems a bit out of whack to me. I had a 22 Hornet that did nearly as well as this 222 Rem. :confused: I have used Barnes load data, Nosler, Hornady, IMR, Alliant.

The chrony has been fairly close with factory ammo specs, and most load data in various calibers.

Questions: Could excess freebore be a major contributor to this?
Are there other factors that might be involved?
 
Yes, too much freebore can affect both velocity and accuracy. Freebore affects the bullet in basically the same way as throat erosion - it gives the bullet a chance to skew before hitting the rifling and also a place for gas to escape around the bullet, which would affect accuracy and lead to throat erosion.

Still, if you are comparing your results to published data, your result may be normal. AFAIK, published data, unless otherwise specified, is taken with a 26inch barrel. Since velocity drop is about 50-70 fps per inch, your results are not too far out.


Jim
 
Two parts to freebore !!
Length of throat will lower pressure .Weatherby catridges have always used this .

Diameter of throat -Many don't ever think of diameter but this will definitely reduce accuracy if cut too big !! The bullet should just ride on the throat not wobble .
 
I had (passed on to son) Ruger .257 Roberts "Centennial Model), that had so much free-bore I could not find a bullet long enough to get anywhere near the lands. I only used hand loads in it, and never saw any signs of pressure no matter how high I went with the loads. However, accuracy with that rifle was dismal (about 1-1/2" to 2 inch groups were the best I could get), but good enough for deer.
It was often said that the reason that the .257 was not more popular in the old days was that the rifles chambered for it were too short-throated to accept the longer bullets for deer. So I suspect the Ruger dealt with that in an excessive fashion.
 
All of the old tang safety Ruger 77s had a lot of freebore. Bill Ruger was determined that no one was going to seat a bullet out against the lands and blow up one of his rifles. That is one reason accuracy was so bad in the older Ruger 77s.
 
Accuracy with this CZ has been so so. With various other varmint rifles, and even a Ruger 25-06, the most accurate loads were the faster loads---not necessarily the fastest, but towards the upper end of the velocity range. This one seems to be underwhelming. The Nosler load data is derived with a 20 inch barrel. I understand that some load data is derived with 24-26 inch barrels. However, this has been less than I would expect.

I have read other threads here where senior members have suggested having a smith remove a barrel, cut off a bit, and reset it to SAAMI spec. This seems to me better than 10 more trips to the range and 500 test rounds that end up repeating the same results. I like to shoot, but this is not getting me cranked up. And trading the gun isn't what I want to do either.

Thanks, everybody for chiming in!
 
Having shot thousands of rounds in MIL SPEC 7.62 NATO chambers whose throat was .3100", bullets with .3082", .3084", .3086" and .3092" all shot about 1/2 MOA at 600 yards. Clearance around those bullets before they entered the barrels with .3000" bore and .3078" groove diameters didn't seem to matter. Therefore, the chamber throat diameter doesn't matter that much to me.

The harder bullets slam into the rifling, the more they'll be deformed before getting out of the barrel. Which is why folks understanding what's important for accuracy want minimum bullet jump to the rifling when it's moving the slowest on its way to the muzzle. Minimum jump to the rifling means the least amount of bullet deformation and unbalance will occur.

A friend I shot matches with years ago worked for Roy Weatherby in the 1950's and designed Weatherby's famous stocks of that era. Roy had his shop make two single-shot 9-lug actions for him and was astonished when the one used for a .300 Wby. Mag. had a standard throat/leade so standard length ammo bullets would just touch the rifling; no jump at all. While its accuracy surpassed any one of the standard Weatherby barrels, it lasted only 700 rounds before the throat/leade eroded too much. But that round receiver twisted too much from barrel torque when fired and had to be rebedded every 150 or so shots.
 
It isn't only the bullet "slamming" into the rifliing that is the problem, it is the fact that freebore (or throat erosion, which is about the same thing but less precise) allows the bullet to skew as it comes out of the case. That not only can distort the base of the bullet but also allow it to skid when taking the rifling. Both conditions contribute to inaccuracy, though the former is worse.

Jim
 
"Slamming" bullets into the rifling has been realized by folks using strain gauges measuring pressure curves. Some powder's start up pressure curves have a steeper slope when the round fires. That starts the bullet out of the case at higher speeds. None of such loads produced the accuracy that powders more gradually building pressure and more gently pushing the bullet into the rifling.

No wonder ball powder's never been popular with folks wanting and getting best accuracy. Extruded powder pushes bullets into the rifling with minimum deformation on impact. Milder primers tend to do the same thing compared to the hot magnum ones.

Some folks think faster twists build up pressure as they present a greater angle of the rifling to the bullet as it leaves the case. There's really only a few degrees of angle difference across twists for a given caliber. And a 1:11 twist in a 30 caliber barrel presents the same angle to the bullet as a 1:8 twist in a 22 caliber one.

Unless the bullet's free of the case mouth for some distance before engaging the rifling, it won't rub on the throat anywhere. The bullet's well engaged by the rifling and centered in the bore before the bullet clears the case mouth as long as its jump to the rifling is about 1/3rd less than the seating depth from case mouth to the bullet's heel.
 
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