Bolt thrust force with the Grendel compared to 223

[EigerWand]

I expect this is well known but I don't really know it.

I'm thinking whether it would be feasible to rebarell a new Sako 85 223 into 6.5 Grendel.

Magazine width, feed and spare metal on the bolt face are all another question but this one is about the margin of safety in the action when you burn 30% more powder and have a bigger cartridge base diameter & area to transmit force to the bolt lugs.

bolt face thrust.jpg

Cart: 223, 6.5GR, 260
Base dia: 0.376, 0.439, 0.4705
Pressure: 52800, 50000,59400
Bolt thrust: 7465, 9636, 13149

Why are the pressures and forces so much less with the 223 and Grendel ?
Is it just because they're rated for semis ?

The load data is taken from the Australian ADI powders handbook online, which should give an unbiased comparison between the cartridges. The loads I compared are the book max with typical heavy for calibre bullets, with ADI's nominal pressure levels.

So, by this simple calculation, the Grendel could generate 30% more force on the bolt lugs than the 223. Is this likely to be a problem ? Actually, is this a good way to compare safety margins ?

[A5BLASTER]

Isnt the sako action the same size of the howa mini?

If so then I don't think it would be a problem. But truthfully I have never seen a sako.

[EigerWand]

It could be the Howa are very similar to the earlier Sakos like the A1.
The 85 is a three lug floorplan with a smooth one piece bolt. I have to admit I haven't had a close look at one in their XS 223 sized action. So this is a more general question about whether a calculated bot force in pounds really indicates safety and whether bolts in general are engineered to have a 30% margin to failure or 300%.

[VASCAR2]

Good questions but one advantage to a Savage conversion is you can get a new bolt head from PTG to mate the 6.5 Grendel case. I’d rather change the bolt head than mill out a 5.56/223 bolt. One advantage to the Cz and Zastava in 7.62x39 was they were chambered for 7.62x39 so was just a barrel change. With the popularity of the 6.5 Grendel Cz now sell three rifles chamber for the 6.5 Grendel, Howa is in the game plus Ruger now has two bolt actions for 6.5 Grendel.

Target Sports has just updated their web site with a Match load from Barnes and two factory loads from Remington in 6.5 Grendel plus PPU and new Hornady Frontier line.

https://www.targetsportsusa.com/65-grendel-ammo-c-928.aspx

[LRRPF52]

Some of the baseline assumptions in that comparison are wrong, probably from being transposed across different measuring systems (SAAMI vs CIP).

.223 Remington working pressure and SAAMI MAP is 55,000psi, whereas the CUP is 52,000.

You need to calculate the bolt thrust with even metrics for the cartridges to get relevant numbers in your comparison.

Last time I did this exercise, .223 Remington and 6.5 Grendel had very comparable bolt thrust.

Since the SAKO actions are one of the toughest ever built using the 3-lug design and Finnish engineering, tested in extreme cold conditions as a regular part of their processes, it's one the least ones that would worry me, especially chambering something like 6.5 Grendel with its low working pressure.

Generally with firearms, you're working with a factor of safety of 2, so the SAKO action is one of the best if someone wanted to push a little.

I used to run a very hot load through a .308 Sako 75 tri-lug bolt hunter that the owner had developed for it with the 155gr Scenar. That thing was a laser even with the shorter hunting barrel, and accurate as sin for such a lightweight rifle.

I've also had conversations with the Lapua engineers in Finland about some of the testing they did with a SAKO action and 6.5 Grendel pushed way above max working pressure using 123gr Scenar. They reached speeds that would scare you, with the Lapua brass coming out with loose primer pockets and deformation to the case head. Brass was toast, but no failures.

The Howa action is a copy of the SAKO L461 basically.

I'm not advocating exceeding book pressures, but the SAKO action will handle pressure well. With Grendel, it's better to let the projectile do the heavy lifting for you with energy retention, and allow yourself that nice sight picture throughout the shot.

[Rosecrans1]

Why are the pressures and forces so much less with the 223 and Grendel ?
Is it just because they're rated for semis ?

To answer this question, it's all about case capacity. The 260's case holds more powder than the Grendel case so it is going to be SAMMI pressure rated much higher. It has little to do in what type gun it's to be operated through.

[EigerWand]

Thanks for helpful advice VASCAR, LRRPF52 and Rosecranz.

I don't like getting figures wrong so will check back from the SAAMI site .
I used the ADI manual to compare psi pressure for known loads that I thought were realistic.
I can see that the industry spec max would reflect more what you could potentially get from factory ammo.

I've heard that brass is the usual limiting component, not bolt strength, but thought I'd start there. It sounds as if bolt lug / action wont be the limitation anyway.

I'll get onto feed, bolt face metal size and so on now.

It's interesting the Lapua people tested their brass to destruction (I use Lapua brass in my Grendel). I'm guessing they used the Sako S sized action, which is sized for 308. I wonder how they got extra hot loads into the case. Mine are compressed as it is and still below book max. The case size seems to limit max loads well before the brass is stressed. I'd be interested if this is general opinion or just my hopeful thinking.

[LRRPF52]

Why are the pressures and forces so much less with the 223 and Grendel ?
Is it just because they're rated for semis ?

To answer this question, it's all about case capacity. The 260's case holds more powder than the Grendel case so it is going to be SAMMI pressure rated much higher. It has little to do in what type gun it's to be operated through.

Only reason I'm highlighting this is for safety and general awareness.

Case Capacity
There are larger case capacities that have way less working pressure specs, and smaller cases like 5.56 that have higher working pressure specs, so you have to look at more than just case capacity.

.260 Rem is a SAAMI MAP of 60,000psi, which is less than what 5.56 is rated for, and bolt thrust is the main reason for that pressure limitation.

Compare .260 Rem with .308 Winchester, its parent, or 7mm-08, and you will see higher working pressures for those cartridges. Why?

When you bottle-neck/overbore the case more, start pressure and bolt thrust increase even shooting lighter projectiles. Bore volume is reduced as well, so things get tight.

With less bore volume to relieve the pressure through for the same case size, the rearward pressure increases (bolt thrust).

Also, if the case has more taper like a .22-250 or 7.62x39, all other variables equal, you get increased bolt thrust due to the shaping of the detonation wanting to act like a rearward-facing shape charge.

Action types
It very much depends on the action and when it was made. Advancements in steel alloys during and after WWII really changed the working pressure ratings for rifles, and rifles like the 1896 Model 6.5x55 Swedish Mausers and Norwegian Krag-Jorgensen rifles are limited in working pressure, whereas ones made with more modern steel actions can take pressure like other modern production rifles.

Vihtavuori even publishes two different data sets for reloading old and new 6.5x55 Mauser actions, with the new being called "6.5x55 SKAN" or "6.5x55 SE", and the old being called "6.5x55 Swedish Mauser".

Another determining factor in the pressure rating for that action is the pressure containment vessel diameters, as well as the bolt lug and lock-up geometry.

For example, the lighter/smaller lever guns have limited working pressures, as do the lighter/smaller single-shot Thompson Contenders.

On the other end, you have actions like the SAKO that are extremely tough and well-engineered to handle modern higher pressures, but the OP is well-validated in asking about the smaller SAKO action chambered in .223 Remington and expanding the bolt face to a larger case head diameter. I will also point out that SAKO has chambered the smaller actions in 6mm PPC, with its 52,000 CUP pressure rating. All of the Hodgdon's pressure figures for 6mm PPC are in CUP, with none higher than 51,300 CUP.

[Rosecrans1]

Thanks for posting this. (Above). When I answered the post, I know I over simplified it. You post covered everything and is much appreciated both for the knowledge and for safety?s sake.

[flyrod]

[EigerWand]

Thanks again, LRRPF52. That's real good having an explanation like that.

Flyrod, yes that was my thinking too. The older A1/L461 action was chambered in 222, 223, 22PPC and 6mmPPC (although few of the PPC were ever sold I believe). The manual says it was rated at 60,000 psi. But the new 85 extra short (XS) action only seems to have been in chambered in 223 so far. Sako have a nicely scaled range of actions, which are a bit easier for me to understand than the great range of bolt actions available from American gunmakers. I'm convinced by your idea on the 284 as well, generating a thrust from the widest part of the case not just the smaller rebated base. I think the Beowulf is rebated like this too but the rated pressure is way down to 33000 psi (from wikipedia) as the bolt strength in AR is the limitation. I wonder if it could be increased in a string bolt like say the Howa mini ?

[Fess]

As you noted, it is best to plan for the worst-case scenario even though brass usually expands enough to grip the sides of the chamber and reduce the thrust seen by the bolt. I know that in the Lee Enfield days, British proof testing of each rifle involved oiling the cartridge to minimize "case grip" and maximize bolt thrust.

[jason miller]

I?ve never owned a 461, but just FYI, the Sako 491 in 6PPC that I used to own, while being scaled in length similarly to my two Howa Minis, had a significantly larger diameter receiver/bolt as well as much bigger/wider lugs on the bolt. Not sure how relevant or useful that info is, but thought I?d throw it out there just in case.

[PGW Steve]

Bolt thrust is the pressure, times the area it is pushing back on, same as a piston in hydraulics. The actions ability to stand up to this pressure is a factor of the lug shear area (and contact), both bolt and receiver/extension and the tensile strength of the material they are made out of. You can model actions in different CAD software packages and run Finite Element Analysis on them and see which ones have the strength.

[kde99]

There is a gentleman on youtube who shoots a Sako that he rechambered in 6.5 Grendel
Video with some detail here - https://www.youtube.com/watch?v=BKuZdC1iEsU
Sako 6 PPC action, Lilja barrel
He's fairly active on youtube, seems to be very responsive to questions