On the Krag, what is the significance of the 22 inch bbl length? Or the 30 inch bbl? Is there something special about either length and the .30-40 military round? Or was it just an arbitrarily chosen length? Other than sight radius would cutting an already cut bbl to 19-20 inches hurt anything? I'm thinking about a scout/krag. Haven't shortened anything yet, still working on the forward scopemount.

Barrel length is important in establishing if a Krag barrel is Springfield Armory work or the product of 'basement gunsmithing'.

Unless I'm missing something, Springfield produced three barrel lengths:

The 30 inch Infantry Rifle barrel, likely arrived at for 'bayonet reach'. (At the time, 29 to 32 inch barrels were kind of standard for Infantry in all Armies of the World).

The 22 inch Carbine barrel, likely found sensible for convenience on horseback and of adequate ballistic performance. (Also, a good length for Pilipino Constables and Military School Boys).

The 26 inch barrel of The Board of Ordnance and Fortifications Rifle. An ultra rare experimental arm that (around 1902) pretty much showed 26 inches is as good as 30 inches.

When the 1903 Springfield rifle was adopted with its approximately 23 & 1/2 inch length barrel, some nations were going to a shorter rifle, that would meet the needs of infantry and cavalry. (The 1903 Springfield and Short-Magazine Lee-Enfield are examples. Both of these short rifles were issued with 17 to 18 inch bayonets to maintain "Reach").

Improvements in propellants and bullet ballistic design likely made 30 inch long barrels totally unnecessary.

For your envisioned 'Krag Scout Rifle', an 18 to 20 inch barrel, should give you adequate performance. With some loads and propellants, a 'flash hidder' (Like on the British No. 5 Rifle, aka - 'Jungle Carbine') might be a good idea.

(p.s. If you ''bob" a barrel, please use one that has already been 'cut-down'. Good original barrels are hard to find. There are lots of cut-downs with excellent bores).

The potential scout/krag candidate has a 1898 action in a 1902 cartouched cut down rifle stock. The bbl measures 22 1/4 inch from the boltface and has a 03 front sight. The stock has an inlet sling swivel and a swivel on the bbl band. But also had a saddle ring mount screwed to the side of the stock, it was never inlet so I just filled in the screw holes with pieces of walnut. Thanks for the bbl info butlersrangers.

Faster powders like 4064 or Varget may give you better performance with the shorter barrels. Handling in brush is improved with shorter barrels.
The down side is muzzle blast. I was with my son standing side by side, me with a 26" barrel '06 and him with a short barrel 223 shooting pigs as they popped out by an opening in a fence. His muzzle blast was deafening compared to the longer '06. Muzzle flash in the scope can be annoying in low light if follow up shots are needed. Putting on a flash suppressor kind of negates the advantage of the shortness.
I have an 18 inch barrel here, if you want to experiment with that length. I used this barrel with light loads of 3031 and 130 grain 30/30 flat point bullets. I do not remember it being too awful on the muzzle report, but my son probably shot it more than I did. At that time the barrel had the laser sight and I held the light while my son went into the field to kill the pigs.

In the days of 30-inch barrels (and other long barreled military rifles), long range musketry was at least a theoretical option.  Also, there was still horse-mounted cavalry.  In pre-gunpowder days, the infantry had pikes to counter the close-up part of a cavalry charge.  Long rifle barrels and a long bayonet gave the individual infantry soldier a form of close-up defense against both horse & rider.

Winchester & Marlin made a lot of .30-30 carbines with 20 inch barrels.  I don't recall any great furor about muzzle blast when those things were/are shot.

I've never been bothered by my own muzzle blast. I have been momentarily blinded by muzzle flash, while hunting in low light and taking a shot, as I sighted through a riflescope.

The 'orange ball' I saw was due to propellant choice. The barrel was 22 inches long. The cartridge was 7X57mm and the propellant 3100.

The experience was like walking out of a dark movie theater into blinding sun light.

(Not complaining, just a factor to take into account).

I don't plan on trying to make a .30-40 Krag into a .308 hotrod. The rifles design has it's limitations. And I have no plans to hunt anymore. So just easy but accurate target loads will be created. But I'm toying with cutting the bbl as I've never liked the 03 sight on a Krag. It doesn't look bad on a full stocked rifle but the nicely sculpted Krag sight is a lot better looking. I probably won't even have any iron sights on this build.

Your gun and your choice; however, if the main goal is to get rid of the 03 front sight, would it not be easier (and more cost effective) to just remove the sight as opposed to cutting and recrowning the barrel? If it is already shooting accurately, why run the risk of losing that by cutting the barrel?

Removed the cross pin and drove the sight base right off with a mallet. No spline cut or step on the bbl. I will hit the small cross pin slot with a little blue and I'm done with the front sight. Looks alot better.