Bulletproof glass blocks are rectangular parallelepipeds, 82-84 mm tall, composed from steeled plates of cast polished reflective 1st category glass or non-steeled plates from the same glass. On a special order, the blocks can be made from barium sulphate glass.
The plates are glued together with synthetic resin created by NII-6.
The block is installed in a steel container produced according to technical requirements for steel containers, attached to these technical requirements.
The gaps between the block and the container are filled with Portland cement according to blueprint #101 and 56-162 (attached).
The blocks are designed to be used in observation devices of fighting vehicles to protect the observer from armour piercing and regular bullets.
3. Technical Requirements
- The light transmission of the block needs to be no less than 65% for regular glass and no less than 80% for barium sulphate glass.
- During inspection with the unaided eye, under sunlight or artificial light, the block must have none of the following within 150 mm of the viewer's eye:
- Harsh mechanical scratches
- Thick string or knotted casting defects
- Chips on the end of the plate wider than 7 mm
If one of the aforementioned defects is discovered, the block is defective.
NOTE: The non-working part of the block is the part between the case and the surface connecting the frames of the rear and bullet-facing parts of the block. The rest of the block is the working part.
- It is acceptable to have the following in the working part:
- Bubbles no more than 0.75 mm in diameter (6) and no more than 0.5 mm in diameter (10) scattered throughout the block. No more than two bubbles on the internal or external surface.
- An air bubble no more than 1 mm in diameter that is no more than 10 mm from the viewport frame. Conditions for bubbles 1 and 2 apply to the glass, as well as the resin.
- Thin string-like defects with surface area of up to 1 mm or small defects visible by unaided eye by shining a light through the block, with the condition that the defects do not impede light transmission and do not cause distortion.
- No more than 5 hair thin scratches up to 25 mm long.
- Light internal mechanical scratches up to 3 mm long or one scratch up to 10 mm long, no more than 10 mm from the frame.
- Light mattness that does not interfere with light transmission.
- Light scratches on the front and rear that do not impede vision.
- It is acceptable to have the following in the non-working part:
- Hair-thin or mechanical scratches
- Scratch or knot defects
- Bubbles up to 2 mm in diameter, as well as bubbles in the resin up to 2 mm in diameter.
- A matte spot on the side of the case caused by the case vice is acceptable no more than 10 mm from the edge of the cap.
- Both vision surfaces of the block need to be parallel. The difference in block thickness needs to be no more than 0.5 mm, and the size must be at least what is specified in the blueprint.
- 2% of the blocks are taken by the BTU military representative for lab trials, and 4% of the blocks are taken for ballistics trials.
Blocks for the the military representative must be provided no less than 3 days after the cement was poured. The military representative receives lab results from the factory laboratory in writing.
- Blocks are subjected to the following trials:
- Moisture tests: the block is submerged in a bath of distilled water at 15-20 degrees for 10 hours. The resin must hold.
- Heat tests: the block is heated to 60 degrees for 30 hours. The resin must not lose hold leak out of the case.
- Light transmission tests: Using a selenium photodiode or a Lummer-Bratkun bench. Light transmission must be no less than 65% for regular glass and no less than 80% for barium sulphate glass.
- Distortion tests: a white screen with a 0.5 mm thick mesh in 10x10 mm cells is photographed. No distortions in the photograph are acceptable.
- Ballistics tests: the block is fired upon with a 7.62 mm B-30 armour piercing bullet at 50 meters at normal, or with a regular VL bullet. The block must be installed in its steel case, filled with cement, and affixed in a turret. Only shots that do not touch the armour or frame count. Ballistics tests must be performed with the military representative present.
After being hit with an armour piercing B-30 bullet or a regular bullet, the rear surface of the block must be whole and have no cracks or fragmentation.
The results of the test are recorded in an act which is signed by the BTU representative and factory representative.
- Blocks accepted by the factory are provided to the military representative with a seal indicating the day, month, and ear of production, as well as the batch number.
The batch must be no less than 200 units, with the exception of situations where a smaller batch was ordered.
- 2% of the blocks are extracted for light, moisture, heat, and distortion tests, no less than 4 units.
2% of the blocks are extracted for tests with B-30 armour piercing bullets and 2% of the blocks are extracted for tests with VL bullets.
The muzzle velocity of the bullets are:
- 840-855 m/s with the regular sharp-tipped bullet.
- 795-810 m/s with the armour piercing bullet.
Blocks are tested with bullets that have an expiry date stated in the manifest. It is acceptable to have a deviation of up to 10 m/s from an average of 10 bullets from the batch. Test muzzle velocity every three months with a chronograph.
- If the blocks fail one or more of the aforementioned trials, secondary trials are held where twice as many blocks are extracted from the batch. Secondary trials only repeat the tests that were failed in initial trials. If the blocks fail trials again, the entire batch is considered defective. Each accepted block is sealed with the military representative's seal and the factory QA department's seal.
- At a temperature of 16-23 degrees and a humidity of 70%, a block can be stored for 3 years. If a block becomes defective before that time, the factory must replace it at its own cost, except in the event of mechanical damage during use or storage.
- Each shipped block is accompanied with a protective glass and instructions for storage and handling. The warranty period of the protective glass at a temperature of 16-23 degrees and humidity of 70% is 6 months.
- Use 1941 production blocks as samples for acceptance testing.
- Blocks that have been accepted by the military representative are wrapped in paper.
- Blocks must be stored at 16-23 degrees at 70% humidity.
- For shipment, blocks wrapped in paper are packed in crates. Soft dry wood shavings are tightly packed in between the rows of blocks and between blocks and the crate to protect the blocks during handling.
- No more than 30 blocks should be placed in one crate.
- Each shipment of blocks should include a manifest placed in one of the crates containing the name of the items, amount, crate numbers, name of the factory director, plant foreman, packer and military representative, instructions for storage, service, and handling, and expiration date.
- Each packed crate is sealed by the factory.