Need input on how much freebore to have on a 404 to work well on a 375 HH conversion 2.85" case with all the bullets on the market.

Getting to the stage of designing the reamer for the 404-375.

Thank you in advance

Personally I don't think freebore is as critical on big bores as on smaller bores. You need to have enough for use with blunt meplat straight sided monos which will require the most.

My own Mauser 404 converted from a 10.75x68 has (don't laugh) almost 7mm of freebore with the standard Kynoch profile 400gr projectiles.
It is a very accurate shooter even with original RWS 347gr projectiles set well back into the case neck to give a good neck grip as they have no cannelure for crimping. With 350gr cast bullets also set back to cover all grease grooves it will print groups of three all cutting each other.

I don't really know what your freebore should measure but I would imagine even 2mm for the heaviest mono (450gr possibly now) you are likely to use would be fine.

The 404 Riml. N.E. (404 Jeffery) throat according to CIP is merely a leade of 1-degree-25'-20"
it starts off at 10.77mm (0.4240") diameter at the step down from case mouth to throat.
Tight! Only 7.62mm travel to "Commencement of rifling" if 10.72mm (0.4220") bullet diameter as specified, and with CIP spec barrel.
CIP still has a screwed up barrel spec for the 404 Jeffery:
10.62mm (0.4181") groove!!!
10.46mm (0.4118") bore/land!!!
Twist: 1:420.00mm (1:16.54")

Most barrels nowadays are .423" or .424" in the grooves.
I like a 1:10" twist.

If it were mine, I would Weatherby-ize the throat and use a faster twist, no slower than 1:14", to make better use of monometals as well as cup&core bullets.

A suggested throat for the 404:

.4240" + .0005" (or .4245"max) freebore diameter
.4150" freebore length, parallel-sided run then taper into rifling with
1.5-degree leade

Letter Rip