What is the optimum weight bullet with this twist rate?

The standard twist rate varies from 1-10” to 1-12”, depending on the maker and the age of the rifle, for the .30-06. So the 10.5” twist should be just fine for bullets that go 100 to 200 grains and up. (It seems most recent barrels are 1-10”)

As for the bullets you chose, only by putting together some loads and shooting them will you know if your particular rifle likes those bullets. The longer all-copper bullets and the ELD style target ones often prefer a faster twist barrel compared to standard profile lead core bullets, so you may find that the rifle doesn’t like them even though the bullet weight you’ve chosen is within the traditional .30-06 range.

Stay safe.

Don't over think this. Virtually all manufacturers make 30-06 with 1:10 twists and have for over 100 years. Some from Europe are 1:11 but are rare and even that slight difference matters little. You're rifle is probably 10 or 11. Never seen a 10.5. Pretty much any 150-220 gr hunting bullet should be fine. Some target bullets much over 210 gr might start to become marginally stable. Target bullets of the same weight tend to be longer and it is bullet length, not weight that really determines twist. I think you'll find a greater difference between bullet brands than weights.

You can experiment with this site to get some ideas.

https://bergerbullets.com/twist-rate-calculator/

nothing spectacular like cloverleafs which I would expect from such a long barrel. The muzzle is .645", so I'm guessing there might be a lot of muzzle whip.

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That long barrel adds 50-75 fps more speed over a standard 22" barrel, but doesn't make it anymore accurate. In fact cutting it shorter would make it stiffer and could increase accuracy, albeit giving up a bit of speed. Back in the day shooters were obsessed with faster muzzle velocity. Which was needed with the poor aerodynamic bullets of the day. The trend today is toward more moderate speeds and count on high BC bullets to retain speed better down range

Oh, and the longer barrel length doesn’t make a rifle more accurate.
If it has open sights a longer sight radius (the distance between the from and rear sights) can make aiming the rifle easier so it may make it easier to hit your target.

If it scoped, the sight radius is moot.

The quality of the barrel, the condition of the barrel, the trueness of the action, the trueness of the barrel attachment and the bedding of the action/barrel into the stock are all big influences on the inherent accuracy of a rifle. Good optics, good ammo and a solid shooter behind the gun are what rally make them shine.

Stay safe.

10.5 inches equals 267 mm. And that is equally goofy, so it's not worth pursuing.

The 'proper' twist rate for about anything is not an 'exact' number, more of a range. Presuming the barrel is originally set up for the 7.65x53mm Argentine round the bore should be .310 or .311 inches, about the same as .303 Brit. You say it's a 30-06; so if it's been rebarreled it may follow .30-06 protocol. As mentioned, .30-06 has a typical twist rate of 1:10. That twist rate works for me for bullets of under 140 grains to 220 grain bullets.

I think the 1:10.5 twist will do for about anything you want. If the barrel is adequate.

Again as mentioned, barrel length does not directly affect accuracy.

That small variation is within tolerances for a 1/10” twist.
Likely the barrel was rifled on a machine called a sine-bar press. This allows a variable adjustable fixture, within a range, to set twist rate. The setting was obviously set close to approximately 1/10”.
As previously stated, exact settings are not critical.

I have a Remington Model-7 in .260Rem. Remington claims a 1/10” twist for the rifle when built. However, by actually measuring, its 1/10.9”. Call it 1/11”. Measured several times...
It shoots “lights out” with 120-123gr bullets to include the 123gr Hornady A-max. Even a long discontinued Sierra 160gr SMP shoots sub moa (5-shots). However, a Hornady 129gr SST shoots “patterns” at 100yds (12” groups, tipped bullets). 140gr SST’s won’t hit the berm at 100yds.

I seriously doubt that your twist rate has anything to do with accuracy. Barrel dynamics, throat and crown will.
Most .30/06 will shoot 150-190gr bullets equally well, but will show preferences.

If you want to see how accurate it can be, try these proven loads, EXACTLY as given...

150-155gr: Sierra 150gr Pro Hunter, Win or Lapua case, Federal 210primer, 48.0gr of H4895. Seat bullet to 3.275”pal. The 150gr Nosler BallisticTip or Hornady 155gr Amax or ELD can be substituted.

165gr: Sierra 165gr Pro Hunter, Win or Lapua cases, Fed 210 primer, 56.0gr IMR4350. For a semi-auto, Win or CCI primers. 168gr Match King or 165gr Nosler BT, or Hornady equivalents may be substituted. 3.300oal.

180gr: same as above except 54.0gr IMR4350. Seat to 3.330” if throat allows, mine all do.

In my experience with different’06’s, (about a dozen), these will give some indication of what rifle is capable of.

For a semi auto, use 46.5gr H4895 with 165gr, 45.0gr with 175-180grbullets. IMR 4895 can be substituted for H4895, but not vice-Versa. IMR4895 is a tad slower than the H.

Quality of barrel, bullets, and Care in ammo assembly most affects accuracy. In that order.

28" sounds like the length of the later modelo Argentine mausers. The earlier rifles also had long barrels. Is it actually an Argentine action AND barrel, just re-chambered for .30US? I believe the later Argentine Mausers had actions long enough to feed the .30US, so rechambering to '06 would be a very simple deal. ? If so, accuracy might come with .310-311" bullets. The bore may not be .308".

To make sure the bigger bullets are safe in the gun, just make sure they slip into a fired case, that was/has been fired in the gun. If the bullet won't enter the case, it won't release the bullet without a lot of extra pressure. "Most" .30-06 chambers have a large enough throat diameter to release over size bullets. I say "most" because many riles in .30-06/.30US I have owned all shot them well, or had large enough throats to release .310-.311", or even .312 diameter bullets.

If the barrel is .310", you might not get good accuracy with .308" bullets, regardless of the load. ? Or not. Just a thought!

Welcome. Get to reloading. Try GooseGestopo's recipe for the 180 grain bullet, but do a switch-ahroo with the Hornady .310" FMJ/BT. (after making sure it slips easily into a case that has been fired in the rifle. That breaks his "exactly rule", but might be close enough. Good luck.

LocoGringo said:

So I did a little experiment today that I have never tried before. The twist rate on a rifle I own was unknown, so I coated about 14" of a cleaning rod with masking tape, attached a cleaning patch to the end of the jag and marked necessary points on the masking tape. After trying 4 times, I came up with the same answer 3 times...10.5" for 1 full rotation. I believe the time I came up with a different answer I over-rotated. That seems like an odd twist rate.

So here's the rest of the story. The rifle is an Argentine Mauser bolt action 30-06 my grandfather built with a 28" barrel. I have no idea when it was built, but my guess is before I was born in the mid-70s. It seems the theory behind the build is to maximize powder burn and velocity for the 30-06, but it's not a heavy barrel profile for prone only shooting. It can actually be shot offhand, although it is heavy and unwieldy due to the length. I don't handload for 30-06 (yet), but I have been disappointed with factory ammo accuracy as well as the left over handloads my Grandfather gave to me. They're adequate for hunting, but nothing spectacular like cloverleafs which I would expect from such a long barrel. The muzzle is .645", so I'm guessing there might be a lot of muzzle whip.

I'm trying to figure out what weight bullet would be best through a long barrel like this with a 1:10.5" twist rate. I'm guessing something like the 175 SMK or 178 SST from Hornady. What do you think?

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It is actually a function of bullet length, not weight. But as diameter is fixed, heavier bullets of the same caliber are generally longer. Still, a 178 gr Hornady ELD-X is probably longer than a 220 gr RNFB.

There is actually a mathematical formula, called the Greenhill Formula, for calculating twist rate:
https://www.titanreloading.com/ballistics-questions/greenhill-formula

But we happen to know from experience that a 1:10 twist .30-06 will stabilize about everything commonly available in that cartridge, from 110 to 220 gr. If the bullet is too long for a 1:10 twist, it is probably a new generation VLD, and the bullet manufacture probably recommends a twist rate on the packaging. For example, I think Hornady recommends a 1:8 for their 225 gr ELD-M.

You might also want to slug your barrel and have a gunsmith check the throat and the crown. If the crown is damaged, it should be easy enough to just chop a couple inches off that 28 inch tube and recrown in. If you have throat erosion, land/groove defects, it might require a new barrel. Someone with a bore scope who knew what they were looking at could answer your questions in a few minutes.