5.56 different throat length for different twist rates.
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DLT
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DShortt
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The rate of rifling twist and the freebore/throat length are not inexorably related. The barrel blank is manufactured with a rifling twist rate. The reamer is manufactured with a freebore/throat length. In order for a particular LENGTH of bullet to be able to function best the correct reamer needs to be used for the correct rate of rifling twist for the length of the bullet. Bullet weights get tossed around a lot but that's not the determining factor. Bullet length is.
Ned Ludd
Silver $$ Contributor
The throat geometry will have a maximum limit that can still allow feeding from a standard or unmodified magazine. In other words, throat length has an upper limit as determined by magazine length, regardless of the barrel twist rate or bullet length. I was not aware that throat length in commercial rifles was rigorously connected to barrel twist rate. I'm not saying it isn't, just that I never really thought about that before. If that is true, then I suspect the reasoning is two-fold. First, a longer bullet often requires a faster twist rate for full gyroscopic stability. Second, the longer bullet typically will benefit from a longer freebore so that it occupies less internal case volume. Thus, loads that generate reasonable velocity can readily be achieved at slightly lower operating pressure than if the bullet shank must seated way down deep in the case.
So it seems to me the reasoning would be as follows: the longer [heavier] bullet would first and foremost be the driving factor for the faster twist rate, to achieve gyroscopic stability. The longer throat [within the limitation of maximum COAL that could successfully feed from a mag] would then allow the longer [heavier] bullet to be loaded at slightly lower pressure. In theory, one could simply use the maximum allowable throat length that would work best with the longest [heaviest] bullets available in commercial ammunition and call it a day.
As an example of that, some company could manufacture .223 Rem bolt rifles with nothing but a Wylde chamber and 7-twist barrels with the expectation that they would thus be optimized for 77 gr Matchkings. Although technically correct, such a setup may not work as well for the buyer that wanted to occasionally shoot 55 or 69 gr bullets because the barrel twist rate was faster than necessary and the shorter bullets might be jumping a considerable distance before encountering the rifling. Nonetheless, that approach might still work acceptably, whereas shooting 77 SMKs from a 9-twist (or slower) barrel with a very short freebore that had been set up for much shorter [lighter] bullets may not work with the 77s at all.
So I think the realistic answer to the OP's question is that the manufacturers might wish to provide their customers some choices with respect to bullet weights and let them decide how the rifle will be optimized to some degree. To that end, a longer throat would generally be expected as barrel twist rate was increased to accommodate longer [heavier] bullets at slightly lower pressure, up to the maximum allowable length that would reliably feed from the magazine. Whether one agrees or disagrees with that business model, it's commonly-used. In the past I have owned a commercial .223 Rem rifle that had a 21" heavy barrel in 10-twist. It would have worked fine as a varmint gun, but I don't. As a target rifle for shooting 400+ yd, it wasn't the greatest choice due the limitations on bullet weight/length. I would never make that mistake again in that given a choice, I would always buy a .223 rifle that came with a 7-twist barrel, regardless of how long it had been throated at the factory. In my hands, 7-twist barrels have worked just fine with commercial ammunition containing bullets as light as 40 gr, whereas the opposite is not necessarily true; 9- or 10-twist barrels often do not work well with 75-77 gr bullets.
I've also been surprised by this having found that table last year.
Here's an interesting take on this issue.
https://www.luckygunner.com/labs/5-56-vs-223/
In Europe, manufacturers of fast-twist factory rifles had a problem for a while in that the EU regulating body the CIP included rifling pitch rate as part of the cartridge and chamber specs and was very unwilling to deviate from the standard based on the original 12-inch pitch. Likewise throat lengths. I well remember testing one of the first 223 Cz527 Varmint models to come into the UK sometime around the turn of the century. It was an excellent little rifle for 'foxing' and suchlike - HS Precision bedding block stock, heavy barrel and (allegedly) 9-inch twist rifling. Loading up a box of 68gn Hornady HPBTMs and Sierra 69gn SMKs, range results at 200 yards were terrible, and it was obviously a lack of stabilisation. Yes, the specs said 9-twist, the actual was the old 12. It turned out CZ had backed down after a disagreement with the CIP - later resolved - and the UK distributor had to take back a lot 527s that had been mis-sold. Freebore appeared as far as I could see to remain as per the original 223 maybe 'stretched' a little.
So when the Tikka 223 T3 Tactical appeared a few years later with an 8-inch twist, I was most intrigued as to what freebore it had been given on the basis that the optimal bullet choice for range use would be 80gn models with freebores allowing 2.45" and above COALs. I never have found out what these rifles came with. I suspect again SAAMI/CIP with a bit of stretching craftily engineered in, but nothing like Wylde. So, the most usable bullet remains the relatively short, blunt 77gn SMK and equivalents from other manufacturers loaded to whatever COAL the magazine allows despite the faster twist rate.
As others say in earlier posts there's no relationship between twist rate and chamber throat lengths, so there is always the likelihood of mismatches which severely constrain COALs and therefore bullets that can sensibly or even practically loaded. The cause is the SAAMI / CIP specs straightjacket, and I really can't see a way around that for factory rifles.
This issue has arisen in spades in the Why do we need the 6.5 Creedmoor? controversy. This forum is littered with posts that basically say words to the effect that with a bit of extra throating and single-loading the existing 260 Rem (for instance) can handle 140s and produce as good accuracy and higher MVs. The point is completely ignored that manufacturers can't make and sell 260 Rem rifles and ammo that are outside SAAMI specs and most LGS customers wouldn't want one either if optimised ammo had to be single-loaded. Therefore, when talking factory rifles / ammo, the hated by so many Creedmoor approach has to be used - an officially new cartridge design with resubmission to SAAMI to have it approved as a standalone cartridge.
Remember that different bullets have an angle where they taper to the tip, some being more or less acute than others.
The angle can be chosen for that angle to give the bullet an accommodating slope for the bullet to transition into the rifling. Think pointy VLDs vs a round nose bullet.
I suspect the angle on the 5.56 is leave more metal for throat erosion and the angle on the 223 for short bullets.
As far as I know, for most things the 1.5 degree is what most target chambers use due to it being a good all around angle.
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