1-9 is a very versatile twist. It should be able to handle the varmint bullets above 40 grains, and keep you entirely stabilized up to 62 grains.
It's not so much the weight of the bullet which matters, but the length. I would think that anything under 77 grain OTM might be just fine, but...Over 62 grain I would be careful.
Remington Model 700 bolt action rifles in 7mm Rem. mag. have a 1 in 9 right hand twist (one full turn in nine inches of barrel). This will accomodate bullet weight/length up to 175 grains.
1- 12 twist
The direction of the spin which is put on the bullet by the rifling.
That would depend on the caliber of the particular Remington Model XP-100 you have. Remington chambered these hand-guns in a multitude of calibers from .221 Fireball and up to the .350 Remington Magnum, so rifling twists will be different from one caliber to the next and the specific twist in that barrel will play a big role in how accurate a bullet performs.
Bullets fired from a rifled firearm should NOT tumble in flight. If they do so, it is a sign of problems. This may be caused by different matters that all relate to the rifling in the firearm, and the bullet. First is rifling is badly worn, if the bore is oversized for the bullet, or if rifling is badly fouled with lead/ jacketing metal/powder deposits, the bullet will not be gripped by the rifling and spun, and may not be stable in flight (tumbling) Try cleaning the bore properly, and check for excessive wear at the muzzle. The second condition is the wrong weight/length of bullet for the rate of twist of the rifling. A fast rate of twist, combined with a very fast bullet, MAY result in the bullet "stripping", and not being spun by the rifling. Or a very heavy, slow bullet may not be spun fast enough to stabilize the bullet. Most .22 rimfire rifles are rifled to shoot 40 grain bullets accurately. If loaded with a 60 grain bullet, such as the Aguilla 60 gr SSS, the bullet will not be stabilized, and may tumble in flight.
In relation to firearms, it has to do with the rate of rifling in the barrel. Rifling is the spiral grooves that puts a spin on a bullet to increase it's accuracy. "Barrel twist" is the rate of spiraling or inches per turn. That is the length of barrel it takes to spin the bullet a full 360 degrees.
The twist on the inside of a gun-barrel is called rifling. I dont know the specific pitch of the rifling in this particular gun but thought you would like to know the proper terminology.
The rate of twist for a Remington 788 in .243cal is 1 in 9 which is one complete turn of the bullet in nine inches
The rifling in the barrel. These are grooves cut on the interior of the barrel that twist around and cause the bullet to spin as it passes down the barrel. The spin stabilizes the bullet and promotes accuracy.
By the twist rate, which is measured in the number of inches per full revolution. For instance, the old original Model 1894s in 30 W.C.F. were made with a rifling twist rate of 12:1. Or in more simple terms, the bullet turns one full revolution in each 12-inches of barrel travel.
Twist is 1:12
The bullet will rotate one full turn in ten iches.