Mikey: I was able to find one industrial source regarding Belmont. It was rather thin at best. But it mention that they specialize in re-manufacturing military ammunition into sub-velocity ammo. If so that could be involved in this scenario.
I do recall the sign at the gunshow giving a velocity of over 2600 fps for this ammo in question. It struck a cord with me because when I bought the rifle in August 1960, I bought two boxes of Golden State remanufacture military ammo with 150 grain softpoints. The velocity was listed at over 2600 feet on the box. It went bang, and kicked the dickens out of me in the carbine.
At the time of purchase my Dad made me take it to the gunsmith to have the headspace checked and to make sure it was safe to shoot. After that I burned up a couple hundred rounds of military ammunition. As a newly wed it became my wifes deer rifle. It kick the dickens out of her.
Along the way I purchased some surplus Canadian military ammo for it. It was the best military surplus ammo I had bought for the .303, it was clean, and the cases could be reloaded. I was already reloading for other calibers and picked up a set of RCBS .303 dies.
According to my Load Book, I started at 34.3 grains of IMR4320 and worked my way up to 41.4 grains of IMR4320. The bullet I was using was the Hornady .312, 174 grains. Comments are "accurate" and "good." All of this load development went on in 1980/81.
According to my notes I used the Canadian Mil. cases, R-P, and Federal cases in my load development.
In the next few weeks I hope to take advantage of the components I have on hand and go back to the 41.4 grains of IMR4320, and the 174 grain Hornaday bullets. I have a couple of boxes of Sierra 180 bullets that I have never developed loads for. I have printed out your post with your successful loads. I currently want to use up the IMR4320 because I have had it for awhile, and it has proven itself. Once it goes dry, then it is time to try something new.
A little footnote on the gunsmith. The fee for the inspection was light, but that was because I had to put up with a two hour lecture on why I should only buy Model 70 Winchesters, which were King in 1960. Nobody made a commercial rifle that was any better. The rifle of choice could only be in 30-06. The gunsmith was a .30 caliber shooter and always went to Camp Perry to work on arms during the matchs. This kid could not afford a M70. But it was his standard lecture. You should have heard him when I bought a .270 Winchester later.
The final load I listed for the .303 British is just below max in the Hornaday manual I used at the time. In another manual it is a ways from max. Yet, in two current manuals it exceeds max. The .303 British has been chambered in a lot of rifles over a many years. Starting out as a black powder round. Follow Mikey's suggestion and get your rifle inspected for safe head space. When loading start at the bottom and work up. Do not start at or near max.