Does anyone have any idea what this is?

Nope. I figured its 65K PSI was plenty of "P" to begin with. Not sure how you'd "+" that.

Where does the reference come from? First I've heard of it.

Rex

Unknown Munitions XLR Rifle Giveaway
This rifle will be displayed at the Hunt Expo in SLC in XLR's booth 5138. The winner will be chosen Saturday 2/17/24 at 3PM MST at the show. You do not need to be present to win.

Unknown Munitions 300 WSM +P Custom Rifle

Bat Machine Medium Vesper Action

XLR Magnesium Element 4.0 Medium 700 Chassis all carbon and all accessories

Benchmark Carbon Barrel 30 Cal 8TW Sendero Lite

TI PRO 4 Muzzle Brake

UM Medium Chassis Magazine

Triggertech Special

UM Premier Scope Rings

UM Complete Cerakote

Looks to me like "+P" is the model of the rifle, not the chambering.

Most likely a reference to a Defensive Edge style custom throat chambering as seen here: https://defensiveedge.net/p-throating-applications/

Men complain about the recoil of a 300wsm so I bet they will loose their minds over this, lol.

This

I appreciate the help, my son won that rifle and I had no idea about the "+Plus"

Congrats to your son. That's a heck of a package.

Load the 300WSM with 180 grain Accubonds and you HAVE a +P accurate load. Ask the 18 plains game animals in my game room. To include a mountain goat, aoudad, red stag, tahr, arapawa sheep and more. The 300WSM is a stout cartridge 5hat needs no stinkin’ +P help. MTG

I LOVE my regular old 300wsm. I prefer 200’s at about 2825-2500fps. They smoke the 180’s after about 300 yards. Everything from elk to kudu to zebra have all ran about 50 yards and fell over dead with zero follow up shots.

Congrats, looks like a heck of a setup.

+P refers to "stepped" rifing in the chamber where the bullet is guided into the rifling in a taper that reduces intial pressure spikes with max loads

pressures are reduced somewhat and more powder is added to bring pressures back up to max, which allows for higher velocities (the +P designation) than what you can get with standard chambers/throats where the bullet hits a wall of rifling that creates high pressures prematurely with certain bullets and powders

usually a 100-200 fps improvement is seen, depending on cartridge, bullet & powders used

I have tested several wildcats with standard vs +P chambers and have always seen an improvement in velocities with stellar accuracy

My 300 Long Range Hunter (300 LRH) wildcat based off the 300 Norma Mag is one that has shown a boost in velocity after rechambering with a +P reamer

So like "Weatherby Free Bore"?

I'd like to see all of these variations and special loads tested in a real pressure lab.

the Wby free bore is a section of the chamber without any rifling, the bullet still has to engage rifling after that and immediately get grabbed by the sudden rifling/twist from no spin into a spin .... this creates the pressure spikes

the stepped rifling is like a rifled funnel that guides the bullet into the rifling/twist without the sudden shock

PRICE LIST

+P EXISTING CHAMBER 200.00


Anyone want to try a $200 experiment?


Would there be any advantage if you only ran factory ammo?

Nope not an advantage for factory ammo and due to the fact that a section of rifling is milled into a taper your throat will be shot out sooner.

I’ve had 2 rifles with +p throats and they do allow more powder to get up to pressure thus produce higher speeds but the trade off in barrel life was a turn off for me.

Not a big deal for guys chasing speed to need to swap a barrel sooner but probably not an advantage for the average shooter/hunter.

Lol

Lot of folks these days rely on bigger/faster instead of their ability to put the bullet where it needs to go.

I rest my case with placement. Period.

Plus P? It's called a 300 win mag.

OK

Now tell me how you're gonna "shot placement" stop a surprise griz charge at 13 yards, you only get two shots, maybe only one